SHEFFIELD, England — They were witnesses at the center of one of Bashar al-Assad’s first monstrous onslaughts against his own people. Marie Colvin, a U.S. foreign correspondent, and her photographer, Paul Conroy, were reporting on the early months of a conflict that would spiral into a brutal civil war.

Trapped in a besieged rebel stronghold, they were caught up in the carnage as the Syrian president’s air force reduced the district to rubble, indiscriminately pulverizing any man, woman, or child left in the neighborhood.

The world was only alerted to the scale of the slaughter because Colvin and Conroy managed to smuggle themselves into the heart of the destruction. Their first dispatch for London’s Sunday Times had raised the alarm, and they planned to file a second story charting the intensification of the killing at the end of the week.

“We knew on the Tuesday that we weren’t going to be alive on the Friday to file,” Conroy told The Daily Beast. “That was a conversation we actually had.”

Rather than flee the massacre, they decided that the only way to grab international attention before most of the remaining residents of the Baba Amr district of Homs had been killed was to go live on TV. With the help of a team of rebel media volunteers, Colvin and Conroy used satellite phones to broadcast to TV networks including CNN and the BBC from the epicenter of an unfolding atrocity. By drawing the world’s attention to the killing, they were simultaneously alerting the Assad regime to their location.

“The media group had been split in half. Six of them had volunteered to stay behind and be martyrs,” said Conroy. “When we went back in, we knew we were going back in to a group of people who had volunteered to die.”

“We said to the guys in the media center, ‘If we do the TV it will bring shit down upon us.’ They said, ‘That’s why you’re here.’”

“The reality was, me and Marie had a discussion—the decision was mutual.”

“ We watched the baby die. We all sat there in silence. I felt rage, actual rage and I know Marie felt the same. And she felt now this was her story and she was going to go for it whatever the cost. ”

Conroy was speaking to The Daily Beast at this summer’s Sheffield Doc Fest after the premiere of Under the Wire, a spectacular documentary that takes the viewer inside a slaughter that killed at least 100 people in Homs, including one of America’s greatest war reporters. Conroy was severely injured and left fighting for his life.

Through a trove of archival footage, interviews and brilliantly conceived reconstruction sequences, the film tells the story of the moment Colvin decided that martyrdom was the only acceptable path left open to her.

The footage from inside Homs includes heartbreaking images of a baby quietly succumbing to fatal injuries inflicted on the orders of his callous president. The staff in a makeshift medical center in Baba Amr have no choice but to stand by and watch the infant’s final breaths. “We watched the baby die. We all sat there in silence,” Conroy says on screen. “I felt rage, actual rage and I know Marie felt the same. And she felt now this was her story and she was going to go for it whatever the cost.”

Lindsey Hilsum, a British foreign correspondent, says she quizzed Colvin over the phone from Lebanon: “I said, ‘What is your exit strategy?’ And she said, ‘There isn’t one.’”

The morning after the last TV interview, the Assad regime directed its missile fire at the building where they were seeking shelter. Colvin, 56, was killed instantly on Feb. 22, 2012, when shrapnel from artillery fire ripped through the doorway.

Marie Colvin, who had worked for The Sunday Times since 1985, applied for a visa to cover the growing conflict in Syria officially, but that request was denied. In early 2012, she and Paul Conroy decided the only way to uncover the true scale of the fighting was to smuggle themselves over the border straight into a warzone.

Hilsum explains in Under the Wire why most journalists had been reluctant to do the same. “I was pretty clear it was beyond the danger threshold,” she says. “Marie said, ‘It’s what we do.’”

The dramatic late-night border crossing is one of the heart-stopping early scenes in the film where director Chris Martin blends footage shot by Conroy at the time, reconstructions, and a voiceover dominated by Conroy’s first-person take on this treacherous journey.

“I’ve heard a bullet. They don’t sound like the films,” he says. “They fucking whizz past ya and you don’t know if they’re near or far.”

After being smuggled through countless vehicles past a slew of checkpoints and regime soldiers, Colvin and Conroy are transferred from safe house to safe house until they are inside a rebel-controlled area. “She just looked at me and said, ‘Paul, we’re in fucking Syria. We made it.’”

These tense action sequences mean the film feels like more of a thriller than a standard documentary. Martin said: “It’s an attempt to try to give people who are watching the film an idea of the unrelenting intensity that Paul and Marie and everybody was going through and, to a large extent, are still going through in Syria right now.”

Tom Brisley, the film’s producer, explained that there was only ever one viable way to tell this story. “For Paul it was an absolute roller-coaster ride. We wanted to tell the story through Paul’s eyes, so it naturally falls into a thriller because it was a thriller,” he told The Daily Beast.

One of the other key figures in the film is Wa’el, a strikingly handsome Syrian rebel who refused payment to work as a fixer for Colvin and Conroy. He described the brief flicker of optimism inspired by the Arab Spring, which had led to rare open protests against the Syrian regime in 2011. “You look around, you think, ‘This is nice, really nice. I think we have a chance,’” says Wa’el. “Then they started sending in these death squads who just opened fire.”

Soon Wa’el, Colvin, and Conroy would be witnessing first-hand the culmination of this bloody crackdown by the regime. “It hit me at that point what the regime was doing to its own people, to its own children,” says Conroy. “I had trouble using the word war. What was it? Slaughter.”

This was the reality they were confronted by once they had been smuggled into Baba Amr in the besieged city of Homs. Again, the director used a mélange of recreated shots and a trove of archival footage that had been found for the movie.

One of the most remarkable scenes of the film shows Colvin and Conroy interviewing people in an underground refuge where dozens of women and young children were huddled in a cramped space known as the “widows’ basement.”

Martin said piecing together this archive of contemporary footage had been a painstaking job with occasional eureka moments. “Somebody said, ‘Oh, I found some footage, it might be of interest to you. It’s Marie and Paul in this basement in Homs and it’s been in my garage.’ So I got the plane to Jordan next day,” he explained.

Terrifying snapshots of video from the deadly bombardment that killed Colvin—as well as the French photojournalist Rémi Ochlik—even include brief glimpses of their devastated bodies.

There is yet more harrowing footage from inside the makeshift hospital where Conroy and French journalist Edith Bouvier were taken after the blast. Both were suffering injuries so severe that they would soon die if they remained trapped in this district under siege.

“I lost hope, definitely. I thought we were going to die there,” says Bouvier on screen.

Amid hour after hour of agonizing waiting, there was plenty of time to capture the atmosphere of the room on film. Conroy initially waved away the cameras that were now turned on him by activists who were helping the lone doctor work with improvised materials to try and keep his patients alive.

Wa’el, who was standing guard over his one surviving charge, recalls: “If you want a definition of misery just go to that room… touch misery by the hand.”

Conroy resolved not to accept death in that crowded, filthy building. “It was quite obvious the cavalry weren’t coming and that left us on our own,” he says. “I’m not going to die in a pool of my own blood, stuck to a mattress in a shithole. I was quite clear that was not going to happen to me.”

After a breathlessly tense sequence in which a team of Red Crescent medics under Assad’s control offer to take the patients to safety, Conroy finally decides “at least we were going to die trying” and opts to make a break for it with his leg held together by an office stapler.

They race through the abandoned streets of the district with gunfire on all sides; Conroy is lowered into the smuggling tunnel they had used to reach Baba Amr. Unable to walk, he is carried through the dark, where oxygen is scarce and there is a “syrupy” atmosphere. Just as we think he is nearing safety, the flashlight illuminates a tunnel collapse—dirt blocks the route to freedom.

“It was just like, oh, come on!” Conroy told The Daily Beast. “That was the cruelest bit of all. Because I knew it was near the end of the tunnel and I just looked up and it was like ‘Nooo!’”

He eventually managed to pull himself through a narrow passage at the top of the blockage, but that meant dragging his badly damaged legs through the rubble, “everything ripping out—all the flesh and the muscle,” Conroy recalls.

He is bundled into a van at the other end of the tunnel and driven away from the carnage. He got his camera back out and filmed his relief. “That’s where my life began again, the back of that van,” he says.

As well as a gut-wrenching thriller, Under the Wire is a morality tale about modern journalism. From the way Conroy describes Colvin’s final assignment, it was a virtual suicide mission.

They succeeded in getting out word of the massacre in Baba Amr, but Colvin lost her life and, despite weeks of horrified media coverage at the time, the civil war continues to rage on in Syria.

Speaking after the premiere in Sheffield, Conroy said he had no doubt that they were right to go back into the besieged city to carry on reporting. “Of course it was the right decision,” he said.

Chris Martin said that his film was about the role of old-school reporters in the modern media landscape. “People going to verify and report and tell the truth about situations is as valid now as it ever has been,” he said, while recognizing that there are limits to what individual journalists can achieve.

“Marie and Paul go in and don’t change the course of events—so is that failure? At least they go and try.”

Martin also examines the character of a journalist who is capable of giving her life in pursuit of the truth. He said he was keen to move beyond the temptation of “sanctifying Marie as a noble warrior.”

She is depicted as dedicated and determined to tell these stories, but also as a relentless competitor and tough woman to work with.

Sean Ryan, a former Sunday Times foreign editor, recalls unsuccessfully trying out Colvin with a number of photographers before she was teamed with Conroy, a former soldier. “One of them told me she was more frightening than the war,” he says.

Conroy said she was often insufferable between jobs, desperate to leave the comfortable life and the parties of London behind to get back on the road looking for her next big story.

When the French team of Edith Bouvier and Rémi Ochlik arrived in Baba Amr a day before the journalists suffered the deadly attack, Colvin was simultaneously fearful for her life and furious that her exclusive was under threat. “She was so rude,” Bouvier remembers.

With a second team of reporters now on the scene, one could argue that it was no longer the sole responsibility of Colvin to tell this story, but she never considered backing away. Conroy says: “She said, ‘You want to leave now that the Eurotrash are here?’ I was like, ‘Fucking hell, Marie!’—It was her story.”

Conroy explained after the screening that it was this competitive drive in both of them that propelled them to take ever greater risks in order to secure the story. “We wouldn’t have been there if it wasn’t that strong for both of us,” he said.

Besides, Conroy explained to The Daily Beast, Colvin was never going to have a conventional retirement. “When I got out, Rich, her partner, turned to me and said: ‘She’d have made a shit old woman, wouldn’t she?’ I said, ‘Absolutely terrible, mate. You’re better off now.’”

In response to the deaths of Colvin and Ochlik, global media companies were keen to back away from their journalists taking such chances, especially now that ISIS and similar jihadi groups were regularly taking hostages.

Conroy was adamant that they shouldn’t stop sending reporters into the world’s most dangerous regions. In the months after his escape, he said he attended meetings with senior figures in Rupert Murdoch’s media empire, which owns The Sunday Times, and he told them “be careful that you don’t make the job impossible by overreacting, don’t throw the baby out with the bathwater.”

“If you stop us going in, then the supply of news dries up and that means they rely on a lot of freelancers. If you’re not prepared to send your own people in, then don’t send anyone else in,” he said.

His leg still flares up occasionally and he had a huge piece of shrapnel removed from his stomach, but Conroy is becoming more used to the camera being turned on him. He is determined to keep telling this story. “Now to go to work,” he said. “I wanted to tell Marie’s story, Rémi’s story and these beautiful [Syrian] people… I’m still doing it and it’s never going to stop.”

‘Under the Wire’ will be in theaters in the U.K. from Sept. 7. It will also be broadcast as part of the BBC’s Storyville strand, and on the History Channel in the U.S.