Before it came for him, Dan Robinson watched the thing grow. It began as a bolus that descended out of the storm, projecting needle-like vortices that lanced the wheat fields near a lone pump jack. Columnar towers a hundred yards wide gathered and darkened against the pale light to the south, unspooling into wispy coronas that moved across the prairie beneath the rotating, two-and-a-half-mile-wide wall cloud above.

It was a little after 6 p.m. on May 31. Dozens of storm chasers were navigating back roads beneath a swollen, low-hung mesocyclone that had brought an early dusk to the remote farm country southwest of El Reno, Oklahoma. Robinson, a website designer and chaser from St. Louis, jumped into his compact Toyota and sped east. He peered out at the tornado framed in his passenger-side window, now wrapping itself in rain so dense that he struggled to make out its leading edge. He swore it was moving farther away. If he got out ahead of it, he reasoned, he might get a better look.

For seven miles, he raced the tornado over dirt roads. It spanned close to a mile, but it would have looked like a shapeless wall of torrential rain to the untrained eye. The last time he'd had a good bead on the funnel, it was tracking east-southeast. Now, as he drove south, he could tell something had changed. It was nearly imperceptible, the way mountains loom larger as you drive toward them. But in 30 seconds, the darkness on the horizon was filling his entire field of vision.

Info Legendary Tornado Chaser Tim Samaras' Last Ride

"I'm getting too close," he said to himself. "I need to start looking for the east [road] option."

An arcing transformer on a utility pole ahead pulsed white light as Robinson bailed east onto Reuter Road. His view to the south was wide open, a country of buffalo grass, red cedar and scrubby blackjack oak. He glanced out of the passenger window but he couldn't find the tornado's outline. That was worrisome. Robinson didn't like getting in front of tornadoes he couldn't see.

He rolled up to Highway 81 and began to pull his car onto the southbound lane, but stopped. The mass was already passing over the road to his south. Robinson drove on across the highway's four lanes and picked up the gravel road on the other side. After no more than half a mile, he caught sight of something out of the corner of his eye.

"What is that?" he said, as a gray, vaporous curtain swept toward the road ahead of him. "What is that?"

At the heading and speed he thought the tornado had been traveling, there was no reason it should be this close. Yet his windshield was lashed by bands of needling rain. A darker form took shape in the south. Robinson blew through the stop sign at Alfadale Road. The heavy rains slackened, and in that moment he knew he should not be there.

"It's just off the side of the road."

The curtain overtook him again and the rain came faster, with a sound against his windshield like stones against glass. His Toyota lurched to the side in 100-mph gusts and began fishtailing in the gravel, causing the car's traction control to cut power to the wheels. He backed off on the accelerator to override it. He did this again and again, never maintaining a speed faster than 42 mph. "The car won't go!" he said.

He punched through swirling eddies of rain. His windshield wipers couldn't clear the water from his windshield. He drove on, blind.

If he had looked at his rearview mirror, he would have seen the headlights of a white Chevy Cobalt, which was somewhere south on Reuter Road. Inside was Tim Samaras, one of the country's most respected tornado scientists, who had built his career by placing sophisticated probes in the paths of oncoming tornadoes. These devices, which he called "turtles," took measurements from inside the storms. No chaser could claim as many intercepts.

Samaras had an uncanny ability for finding twisters and for escaping them with his life. But the monster hiding in the rain that day was something he had never encountered before. What neither Robinson nor Samaras could have known was that in seconds it had grown from a mile to 2.6 miles wide, making it the largest tornado ever documented. And it was tearing toward them across the wide-open wheat fields at highway speed. The difference between escape and incomprehensible violence was measured in hundreds of yards on Reuter Road. And while Robinson never looked back, his rear-facing dash camera did, capturing the last living images of a legend.

To ride with Tim Samaras and his expert forecaster, Carl Young, was to ride with the "big boys," as Matt Grzych puts it. For two seasons Grzych ventured with them beneath mesocylones, the rotating masses of air that stretch for miles overhead and often spawn tornadoes. In a white crew-cab GMC truck outfitted with a winch, chain saws and a mobile weather station, they'd run them down.

He remembers the way that truck could slice through the current of rain, hail and wind feeding the super-cell thunderstorm. They'd drop down ahead of the tornado, deploy devices made of hardened steel and filled with instrumentation to measure wind velocity, barometric pressure and temperature. Then they'd run as fast as the GMC could carry them.

Much of this was well-documented on the Discovery Channel's Storm Chasers. But it only told part of the story. Samaras and Young were one component of a much larger endeavor. Left out was the rest of TWISTEX, a loose confederation of Ph.D.s, trained spotters and meteorologists who fanned out behind the tornadoes in Chevy Cobalts, assembling themselves into a dragnet of atmospheric measurements. As important as it was to get readings from inside tornadoes, they also needed to understand the environment that caused them to form, intensify and unravel. But that part of the operation didn't make for good TV. So the camera crew focused on Grzych, Samaras and Young, and their daredevil tornado intercepts.

The chasers were willing to get close enough to smell ripped-up grass or the scent of splintered lumber and shredded insulation given off by the twister. Once, when they ventured into Dixie Alley and found a tornado hidden inside the deep pine woods near Canton, Mississippi, Grzych pleaded with them to stay out of the trees. But Samaras had already announced that they would deploy a probe at all costs. They narrowly missed a tornado that felled timber and power lines as it crossed the road no more than a hundred yards in front of them. As Samaras surveyed homes scoured from their foundations, he told the cameras and his colleagues that this was why they chased — to feed hard data into the study of these dimly understood and deadly phenomena. The risks, for him, were worth it. Yet they were carefully calculated, and he had always managed to bring his crew out alive.

Samaras, a slight, professorial-looking man with an aquiline nose and kind eyes, was an autodidact with only a high school education. He nonetheless went on to become a star engineer at Applied Research Associates in Littleton, Colorado, specializing in blast testing and airliner crash investigation. The National Transportation Safety Board recognized him for his work on TWA flight 800, which exploded over the Atlantic Ocean in 1996, killing 230 passengers.

Samaras loved a puzzle, to know how things worked. And there were few greater mysteries than the titans that tore through the plains east of his home in the Colorado foothills. Samaras had nursed an interest in tornadoes since he was a boy, when he first laid eyes on the twister in The Wizard of Oz. He began chasing in his 20s, wanting only to be near them, transfixed by their terrible beauty, and by the sounds and the way they smelled. When experiencing the tornadoes was no longer enough, and his analytical mind sought questions that his eyes couldn't answer, his engineering ability and resources transformed a passing fascination into a legitimate scientific pursuit. Using a wind tunnel, he developed turtle probes that remained firmly anchored to the ground even as they took a direct hit.

They were put to the ultimate test on June 24, 2003, a couple miles north of tiny Manchester, South Dakota. Samaras jogged into a roadside ditch, hefting a probe as an EF-4 tornado bore down on him. Moments later, the tornado struck the instrument. Samaras watched from a safe remove as houses were blown apart like piles of leaves. The tornado that razed Manchester registered the steepest drop in barometric pressure on record, and it was captured on Samaras' turtle.

The finding catapulted him to fame, and Samaras seized the opportunity to advance his work. National Geographic wanted to underwrite his research. He partnered with the University of Iowa's famed tornado laboratory. Boeing paid him to field-test hail-resistant skin for its aircraft. He found a chase partner in Carl Young, a bit-part Hollywood actor turned atmospheric science student who was quickly becoming a promising forecaster. He began collaborating with Drs. Bruce Lee and Cathy Finley, University of Northern Colorado researchers who studied the forces at work outside of tornadoes. TWISTEX was born.

The group authored peer-reviewed papers for Monthly Weather Review and the American Meteorological Society. They could lay claim to nearly every measurement taken from within a tornado. This was partially because Samaras was a brilliant engineer, but it was also because no one could read a storm quite like he could. Young excelled at choosing the right storm systems using Doppler radar, but once they sat beneath the mesocyclone, Samaras' ability to spot the signs led them to the tornado.

Samaras was an aggressive, dogged chaser, who often had to be reminded by his colleagues to stop and eat. But he was also beloved. To his children, he was the father who set up a tripod camera in front of the Christmas tree because they had demanded evidence of Santa's existence. Something "unexplained" usually happened as it filmed. He once dressed his son Paul as a ham radio for Halloween. Among Samaras' achievements, he was the first male Girl Scout troop leader in Colorado.

To his chasing friends, he was the guy who had them out to his home in Bennett, where the Great Plains met the foothills, for war stories and copious bowls of his "bunghole-burnin' green chili." He was the vaguely superstitious, empirical scientist who left a McDonald's cheeseburger on his dash every season as a sort of tornado-locating talisman. "They were probably as hard as hockey pucks by the end of the season," says TWISTEX team member Ed Grubb.

To his colleagues, he was their benevolent leader and mentor. Chris Karstens, a Ph.D. at the National Severe Storms Laboratory in Norman, Oklahoma, was just a young University of Iowa meteorology student when he encountered Samaras' unflappability. A fuse had blown on the converter powering Karsten's laptop. A storm churned above.

"We didn't know how to fix it, and I was pretty new," Karstens recalls. "He came back and he was very calm and chipper and came in there and fixed it in 10 seconds. It was just the most bizarre thing. Most people are stressed out and angry, but he wasn't."

Samaras made sure his crew ate well and stayed in the best lodging to be found in the one-stoplight towns they passed through. But every chaser will tell you the pursuit exacts a price. For days, sometimes weeks at a time, they leave loved ones and place themselves at hazard — in part because they want to better understand the storms, but also because men have always taken the measure of themselves against the natural world. Though he respected these forces, by walking away with his life from hundreds of tornadoes, in some way Samaras had shown he was equal to them.

"You have to wonder, because people liken it to some supernatural force," Karstens says. "Did he get away with seeing that thing too many times? Was it just too much contorted in one way that it had to take something back at some point? I don't know."

After the 2011 tornado season, the Discovery Channel canceled Storm Chasers, and with it a significant source of funding for TWISTEX. The next year, one of the weakest seasons on record, the team was all but dormant. But as 2013 rolled around, Samaras managed to secure a grant through National Geographic for lightning research. As a ballistics researcher, he'd used a one-ton camera capable of capturing 150,000 frames per second to study explosions. When the government put it up for auction, he bought the hulking device for $600. Samaras replaced the film technology with digital sensors that allowed him to capture up to a million frames per second. The "Kahuna," as it came to be known, sought the moment of contact when intricate, negatively charged fingers of light splintered out of the sky, meeting a positive charge reaching up out of the earth. Samaras pursued yet another of nature's most fleeting moments.

For now, his tornado research would remain on the back burner. Samaras brought his 24-year-old son Paul, a Star Wars geek who'd developed into a brilliant photographer and videographer. And he brought his trusted chase partner Carl Young. They crisscrossed the Corn Belt together, hunting lightning. If they chased twisters, it would be on their own time and on their own dime.

On May 19, Matt Grzych sat in gridlocked traffic in Moore, a suburb of Oklahoma City, during a stalled chase. A mile-wide EF-5 tornado tore through the middle of town and across Interstate 35, uprooting sturdy oaks and shearing houses from their foundations. The elementary school near him was razed, killing seven children. Grzych watched as those around him panicked. Trucks sped through the median, some in reverse, while insulation rained down out of the sky. It was the first EF-5 he'd ever witnessed. He swore he'd never chase in the Oklahoma City metro area again.

Almost as soon as he'd posted about his experience on Facebook, he heard from an envious Young. "He called me up immediately, freaking out about how I got onto Moore," Grzych says. "His main thing was, 'What were you looking at in the forecast that brought you to Moore?' Carl was all about big tornadoes." Yet he'd never witnessed the strongest: For all their talent for finding tornadoes, neither Young nor Samaras had ever encountered an EF-5.

Eleven days later, violent super-cell thunderstorms were forecast near Oklahoma City. Samaras, Paul and Young met Cathy Finley and Bruce Lee in Guthrie, 30 miles to the north. They'd arrived in the Cobalt, with three turtle probes in the trunk, leaving the Kahuna back in Kansas. Looking back, some of Samaras' colleagues were surprised by his decision to use the Cobalt to attempt to deploy a probe. The four-cylinder, two-wheel-drive sedan would have been weighed down with three grown men and three heavy probes. Tony Laubach, a TWISTEX team member who'd driven one, likened it to a pizza delivery car. "It did fine," he said. "I chased with it for many years. But it didn't handle some roads so good. It didn't handle high winds."

It was, however, economical, and TWISTEX operations were on a shoestring.

Young was a little frustrated, Finley recalls. They'd missed a strong tornado a few days before because of Samaras' research obligations, and Young was itching to see one. They weren't about to miss the setup forming over Oklahoma, predicted to explode the following day. But Finley and Lee told them they would not be joining them for this chase. They were wary of pursuing tornadoes into densely populated areas. As they'd all seen in Moore, the roads tended to get clotted with panicked people and the growing ranks of amateur storm chasers. They wished their friends luck, and watched the towering clouds decay in the sunset.

Inside the nerve center at the National Weather Service Forecast Office in Norman, Oklahoma, a team of meteorologists sat around a horseshoe-shaped desk, peering into monitors, their faces bathed in the primary colors of Doppler radar imaging. Along one wall, a battery of flat-screen televisions was tuned to The Weather Channel and local news. Despite the boiling in the atmosphere west of Oklahoma City, the room was quiet.

Meteorologist Jonathan Kurtz saw a complex system of storms merging, and he needed to know where they were headed. Warm, dry air was blowing out of the Rocky Mountains and rising in their lee, leaving a void of low pressure. Warm, moist air from the Gulf of Mexico was rushing into the void along this imaginary boundary, known as the dryline, which happened to be sitting right over Central Oklahoma. The Gulf air wanted to rise, but it was being blocked by a cap of dry desert air.

The atmospheric instability was building. Once it was warm enough near the surface, probably by late that afternoon, the Gulf air would punch through the cap. Soon, it would meet the cold, 85-mph jet stream from the north. At the same time, the vacuum created below by its rising would draw strong southerly winds. The differences in wind speed, elevation and direction of these two currents, known as wind shear, were getting ready to set this unstable air mass spinning. That was the stuff of all super-cell thunderstorms. What alarmed the forecasters was the off-the-charts strength of its ingredients. Kurtz knew something big was about to happen on May 31.

Samaras and Young lost sight of the tornado in the rain as they drove east down Reuter Road. Approaching the intersection at Choctaw, they would have known at least that it was a mile to a mile and half to their south, bearing east-southeast. They were in position. This was how they operated, parallel and northeast of the storm. When they pulled up to the intersection, they would have seen Dan Robinson driving north down Choctaw, then turning onto Reuter ahead of them.

After a mile, as Robinson paused at Highway 81, he would have seen them pull up right behind him, along with the gauzy curtain of the tornado's outer circulation looming in the south. Because Young's camera was later found, we know a little about what transpired in that car for roughly 12 minutes, until the final minute or two.

Samaras took a call from a reporter as Young steered along the dusty back roads. Young seemed annoyed: Samaras was supposed to be the navigator, and Young needed to know what the roads ahead looked like; they had a habit of dead-ending unexpectedly. Samaras rushed the reporter off the phone, and they began discussing their next move.

Again and again, Samaras told Young to slow down and let the tornado get ahead of them, worried it might cut them off somewhere down the road. But Young wanted to get further east, to deploy a probe ahead of it. Samaras, who always made the final call in deployment situations, didn't override him.

They commented on how poor the visibility was becoming. They sounded confused, disoriented. Samaras said he wasn't sure he could see the funnel anymore.

But it was still there, growing, hooking to the northeast and doubling in speed. It's likely they were in its outer circulation, though they almost certainly didn't realize it. Because Young put his camera down on the floorboard, there was only the sound of heavy rain, wind and their voices. No one in the car was panicking. At the end of the video, perhaps a minute or two before the tornado overtook them, Samaras said in a matter-of-fact tone: "We're in a bad spot."

Robinson's rear dash cam tells the rest of the story. At 6:20 p.m., as Robinson fled, the thin, drifting miasma gave way to something opaque and iron-gray, moving toward the road from the south. Headlights behind him appeared to fall back a half a mile or so. They shrank farther and farther into the distance. As Robinson was pummeled by rain bands and 100-mph inflowing winds, the camera lost track of them.

A few moments later, Samaras' car crested a rise, and was seen as little more than two points of light in the gathering dark. For the first time, it was as though the tornado had shed the cloak and offered the men a glimpse of itself. Its outline stood sharply against the dim horizon. Sub-vortices roped around a mile-wide column and vanished behind it.

But in a matter of seconds, it swelled to 2.6 miles wide and its sharp edges were lost again in currents of rain. As it closed in at up to 60 mph, everyone in that car likely knew what was about to happen. Paul probably trained his video camera on the tornado right up until the very end, members of TWISTEX say. But that camera was never found.

In the last existing images of the three men alive, their headlights shone brightly as the clouds above lowered and a dark wall swallowed the horizon. They were obscured for a moment by a sheet of rain running down Robinson's rear window. They reappeared as the faintest of lights and glimmered once more. Then, in an instant, the wall moved into the road and they were extinguished.

Ahead of them, the way before Robinson cleared. Behind, through the rain-streaked window, there was nothing — no gravel road, no trees, no wheat fields, no sun or sky. It was as though the world had ended there. Nearly veiled in the tornado's half-light, brighter shapes emerged from inside and spun across the face of it.

Just south of the road, there was a thin band of sky between the mesocyclone and the earth. The ragged trailing edge was back-lit in the sun. Below it, the inflowing air feeding the tornado raced low over the plain like smoke in an impossible wind. Robinson stopped 400 yards away and stepped out of the car and looked on the thing that had nearly killed him. The post oaks along the road bowed toward the tornado as the storm drew the wind to its core. He would always question what he did next, and what he had done that day. He would come to see differently the act of stopping, pulling his video camera from the back seat and crow-hopping with the 80-mph gusts at his back, tearing a shoe from his foot. He knew he had gone out that day and met some other thing that he was not equal to. He knew it when a 2-inch hailstone opened up a bleeding gash over his left eye. He knew it when he was sheltering in the ditch and the tornado's outer circulation shattered his Toyota's rear window and waylaid the world around him.

Once the nickel-size hail had passed, Sergeant Doug Gerten of the Canadian County Sheriff's Office got out of his SUV to investigate a car sitting in a canola field, northeast of the intersection at Reuter and Radio roads. He knew it was a car only because it had a single wheel left, with the Chevy emblem on the hubcap. Otherwise, it was unrecognizable, as though it had been cubed by a salvage yard's compactor. "There wasn't a straight piece of metal on it," he says.

He could see that there was a person inside, still wearing his safety belt. He confirmed the man was dead and removed his wallet and took out the driver's license. Gerten watched Storm Chasers, and he knew exactly who Tim Samaras was. As he began his search, he found the Cobalt's motor half a mile away. He noted gouges in the wheat field south of Reuter, where the car had been driven into the soil.

Judging by where the debris field began, the car had been carried nearly half a mile before it was dropped vertically on its rear end. Somewhere in between, deputies found Young in a ditch. Paul's body wouldn't be located until early the next morning. The fire department cut Samaras out of the Cobalt, and a wrecker hauled it off. Gerten met Kathy Samaras a few days later. She had traveled from their home in Colorado, to see where her husband and her only son had died.

"You've got to admire the lady," Gerten says. "She's held up better through this than I would have."

At a memorial in Littleton, Colorado, she said she didn't know how she was still standing.

From time to time over the next month or so, Gerten drove down that stretch of Reuter, looking for the equipment he knew must still be out there. On July 3, he caught sight of a small black object, half submerged in the creek. He stopped, clambered down into water that was only a few inches deep, and came up with Young's camera.

The following day, Gabe Garfield of the National Weather Service set out from Norman with a team to pore over a savaged landscape. He found, however, that little had actually been damaged, primarily because the tornado had passed through the unpopulated farm country. What wreckage in its path he did find merited the twister a middling EF-3 rating. Yet for all the drama of ruined homes and broken trees, the most incredible evidence he saw was in high-resolution Doppler images collected by the University of Oklahoma's RaXpol mobile radar system.

Most tornadoes of that size maintain a fairly straight heading and make a left turn as they weaken. This tornado arced to the southeast, riding the southern edge of the mesocyclone. It was then slung-shot sharply northeast, growing in size, speed and intensity as it turned. It became so powerful that it pulled the tornado cyclone — the wall cloud itself — to the ground sometime after it crossed Highway 81.

The 2.6-mile-wide wedge was incredible, but its winds weren't all that powerful. Inside of it, though, were swarms of sub-vortices, 200-yard-wide tornadoes within the tornado, whose wind speeds approached 300 mph. Combined with the way it wreathed itself in rain drawn from the mesocyclone it orbited, this tornado, in the words of veteran chaser Amos Magliocco, "was designed to kill storm chasers."

Garfield believes that from their position to the north of the tornado, Samaras, Paul and Young didn't see it coming through the rain until it was too late. "I did the calculation. If you're spanning from a mile to two and a half miles wide in five minutes, it adds another five to 10 mph to your effective speed. So, if you're talking 45- to 50-mph actual storm motion, what you're ending up with effectively is a 55- to 60-mph closing speed. That's highway speed that the edge of the tornado is coming at you, and your expectation is for speeds of 20 to 30 mph. If you think you have five minutes based on what your expectation of the scenario is, you actually only have two and a half minutes to get out of there."

Add to this the unfathomable velocity of sub-vortices the size of two football fields, and Samaras' position to the northeast of the tornado was not survivable. "In terms of the sub-vortices' closing speed: 150 to 200 mph," Garfield says. "You don't have time to respond to that. It would literally be there and you would not know."

On a recent afternoon, beneath a wide dome of sky over the Southern Plains, untroubled by clouds, a stretch of Reuter Road still bore signs of violence. Barbwire lay in coils in the ditch. Steel fence posts laid bent and flat against the earth. A single headlight, the kind belonging to a sedan, sat just off the road. Pieces of metal and glass glinted in the field to the south, where the car would have been carried by the counter-clockwise rotation of the tornado. Nearly three quarters of a mile down, on the other side of the road, a car's white bumper lay in the waist-high grass.

Close by, a stained wooden board had been driven into the ground and etched with initials: TS, PS and CY, all arrayed around a pair of wings with a twister in between. It said: R.I.P., TWISTEX, 5-31-13. Next to it was a bouquet of silk daisies and roses, a tiny American flag and a car's gray floor mat. For an hour or so, not a single car or truck passed through this remote stretch of road. There was only the sound of the wind blowing down out of the northeast.

Matt Grzych will always wonder why Samaras, Paul and Young were in that place at that moment. Were the winds and the weight of three men too much for the Cobalt? Did the engine fail? Did they blow a tire? Or had they simply been playing the odds for too long?

"Everyone had that false impression in their minds, that we're too good, that we'll always beat it," he says. "As humans, we think of it as a solid object. We plan our actions around a solid object. But they're ghosts. They're in one place and can appear in another."

Their deaths have forced the insular storm-chasing community to search its soul. None from their ranks had ever died in a tornado. And this wasn't some amateur yahoo with an iPhone. Samaras was the godfather of this pursuit. Now he and the compacted hull of his white Chevy Cobalt had become the glaring evidence of their own fallibility. If so great a man could not save himself, how could any?

Yet Dan Robinson had saved himself, a fact that had not ceased to puzzle him. He had stopped and filmed the thing as it passed, barely out of its reach. He should have been poring over the incredible, once-in-a-lifetime footage his video cameras had captured from within the outer circulation of a tornado. But he couldn't bring himself to look at any of it for days.

When he finally saw those headlights, Robinson was plagued by the same questions that plagued Grzych. "I've thought about this hundreds of times," he says. "I can't imagine they were doing anything different than me. I wonder why they slowed down and got so far behind."

He's haunted by the blind randomness of it all. Had the tornado's arc been just a degree wider, he isn't so sure he would have survived. Reuter dead-ended at the next intersection. He was about to run out of road.

"There's always been chasers who pushed the limits, got too close, and I've certainly done that a few times myself," Robinson says. "You'd think maybe it should have been somebody who did something reckless or careless. It shakes you up when you realize that someone with his experience can end up in that situation."

One of the things Samaras loved about the study of tornadoes was that it remains a wide-open frontier. So many fundamental questions continue to go unanswered. How much can the pressure fall inside of a tornado? Why do some mesocyclones produce tornadoes while others do not?

And perhaps that's what is so maddening about what happened to Carl Young and Tim and Paul Samaras, for those who knew them and for those who survived. There is no simple explanation, no single factor. As unknowable as the chain of random events that give rise to tornadoes is, so too was the series of decisions that ended three lives.