The fire moved quickly through the house, a one-story wood-frame structure in a working-class neighborhood of Corsicana, in northeast Texas. Flames spread along the walls, bursting through doorways, blistering paint and tiles and furniture. Smoke pressed against the ceiling, then banked downward, seeping into each room and through crevices in the windows, staining the morning sky.

Buffie Barbee, who was eleven years old and lived two houses down, was playing in her back yard when she smelled the smoke. She ran inside and told her mother, Diane, and they hurried up the street; that’s when they saw the smoldering house and Cameron Todd Willingham standing on the front porch, wearing only a pair of jeans, his chest blackened with soot, his hair and eyelids singed. He was screaming, “My babies are burning up!” His children—Karmon and Kameron, who were one-year-old twin girls, and two-year-old Amber—were trapped inside.

Willingham told the Barbees to call the Fire Department, and while Diane raced down the street to get help he found a stick and broke the children’s bedroom window. Fire lashed through the hole. He broke another window; flames burst through it, too, and he retreated into the yard, kneeling in front of the house. A neighbor later told police that Willingham intermittently cried, “My babies!” then fell silent, as if he had “blocked the fire out of his mind.”

Diane Barbee, returning to the scene, could feel intense heat radiating off the house. Moments later, the five windows of the children’s room exploded and flames “blew out,” as Barbee put it. Within minutes, the first firemen had arrived, and Willingham approached them, shouting that his children were in their bedroom, where the flames were thickest. A fireman sent word over his radio for rescue teams to “step on it.”

More men showed up, uncoiling hoses and aiming water at the blaze. One fireman, who had an air tank strapped to his back and a mask covering his face, slipped through a window but was hit by water from a hose and had to retreat. He then charged through the front door, into a swirl of smoke and fire. Heading down the main corridor, he reached the kitchen, where he saw a refrigerator blocking the back door.

Todd Willingham, looking on, appeared to grow more hysterical, and a police chaplain named George Monaghan led him to the back of a fire truck and tried to calm him down. Willingham explained that his wife, Stacy, had gone out earlier that morning, and that he had been jolted from sleep by Amber screaming, “Daddy! Daddy!”

“My little girl was trying to wake me up and tell me about the fire,” he said, adding, “I couldn’t get my babies out.”

While he was talking, a fireman emerged from the house, cradling Amber. As she was given C.P.R., Willingham, who was twenty-three years old and powerfully built, ran to see her, then suddenly headed toward the babies’ room. Monaghan and another man restrained him. “We had to wrestle with him and then handcuff him, for his and our protection,” Monaghan later told police. “I received a black eye.” One of the first firemen at the scene told investigators that, at an earlier point, he had also held Willingham back. “Based on what I saw on how the fire was burning, it would have been crazy for anyone to try and go into the house,” he said.

Willingham was taken to a hospital, where he was told that Amber—who had actually been found in the master bedroom—had died of smoke inhalation. Kameron and Karmon had been lying on the floor of the children’s bedroom, their bodies severely burned. According to the medical examiner, they, too, died from smoke inhalation.

News of the tragedy, which took place on December 23, 1991, spread through Corsicana. A small city fifty-five miles northeast of Waco, it had once been the center of Texas’s first oil boom, but many of the wells had since dried up, and more than a quarter of the city’s twenty thousand inhabitants had fallen into poverty. Several stores along the main street were shuttered, giving the place the feel of an abandoned outpost.

Willingham and his wife, who was twenty-two years old, had virtually no money. Stacy worked in her brother’s bar, called Some Other Place, and Willingham, an unemployed auto mechanic, had been caring for the kids. The community took up a collection to help the Willinghams pay for funeral arrangements.

Fire investigators, meanwhile, tried to determine the cause of the blaze. (Willingham gave authorities permission to search the house: “I know we might not ever know all the answers, but I’d just like to know why my babies were taken from me.”) Douglas Fogg, who was then the assistant fire chief in Corsicana, conducted the initial inspection. He was tall, with a crew cut, and his voice was raspy from years of inhaling smoke from fires and cigarettes. He had grown up in Corsicana and, after graduating from high school, in 1963, he had joined the Navy, serving as a medic in Vietnam, where he was wounded on four occasions. He was awarded a Purple Heart each time. After he returned from Vietnam, he became a firefighter, and by the time of the Willingham blaze he had been battling fire—or what he calls “the beast”—for more than twenty years, and had become a certified arson investigator. “You learn that fire talks to you,” he told me.

He was soon joined on the case by one of the state’s leading arson sleuths, a deputy fire marshal named Manuel Vasquez, who has since died. Short, with a paunch, Vasquez had investigated more than twelve hundred fires. Arson investigators have always been considered a special breed of detective. In the 1991 movie “Backdraft,” a heroic arson investigator says of fire, “It breathes, it eats, and it hates. The only way to beat it is to think like it. To know that this flame will spread this way across the door and up across the ceiling.” Vasquez, who had previously worked in Army intelligence, had several maxims of his own. One was “Fire does not destroy evidence—it creates it.” Another was “The fire tells the story. I am just the interpreter.” He cultivated a Sherlock Holmes-like aura of invincibility. Once, he was asked under oath whether he had ever been mistaken in a case. “If I have, sir, I don’t know,” he responded. “It’s never been pointed out.”

Vasquez and Fogg visited the Willinghams’ house four days after the blaze. Following protocol, they moved from the least burned areas toward the most damaged ones. “It is a systematic method,” Vasquez later testified, adding, “I’m just collecting information. . . . I have not made any determination. I don’t have any preconceived idea.”

The men slowly toured the perimeter of the house, taking notes and photographs, like archeologists mapping out a ruin. Upon opening the back door, Vasquez observed that there was just enough space to squeeze past the refrigerator blocking the exit. The air smelled of burned rubber and melted wires; a damp ash covered the ground, sticking to their boots. In the kitchen, Vasquez and Fogg discerned only smoke and heat damage—a sign that the fire had not originated there—and so they pushed deeper into the nine-hundred-and-seventy-five-square-foot building. A central corridor led past a utility room and the master bedroom, then past a small living room, on the left, and the children’s bedroom, on the right, ending at the front door, which opened onto the porch. Vasquez tried to take in everything, a process that he compared to entering one’s mother-in-law’s house for the first time: “I have the same curiosity.”