The image of a wounded pregnant woman rushed out of a Mariupol hospital bombed by Russia showed the world the horror and brutality of the Russian invasion of Ukraine. The March 2022 photo instantly became a defining photograph of the war.

When Associated Press video journalist Mstyslav Chernov, photographer Evgeniy Maloletka and field producer Vasilisa Stepanenko captured these images, they were the last international journalists left in the besieged city of 400,000. Realizing that they were witnesses to history and perhaps war crimes — Chernov set out to do more than collect still images and short clips for the evening news.

“I started to turn off my camera less often,” he said. “But I kept running out of batteries and always wondered whether I would have enough space to store everything. And I kept asking myself, ‘what can I do with all this material?’”

The 30 hours of footage he smuggled out of Ukraine has been crafted into 20 Days in Mariupol, a FRONTLINE-Associated Press documentary that premiered at the Sundance Film Festival and won the coveted audience prize for world cinema documentary. Largely produced at GBH, the film is FRONTLINE’s first to premiere at Sundance.

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Courtesy of Mstyslav Chernov / AP

Narrated by Chernov, the documentary is an unflinching, harrowing account of the invasion and Chernov’s and his colleagues’ eventual escape.

“I wanted to show people the intensity and the amount of pain and suffering concentrated within a very short period of time,” said Chernov. “To do that I needed to build something larger, which was out of my experience at the time.” Together with FRONTLINE editor Michelle Mizner, he created a film that is at once personal and journalistic.

Chernov insisted that more than telling his personal story, the film captures the experiences of the Ukrainian people.

A conflict reporter who has covered wars in Iraq, Syria, Libya, Gaza and Ukraine, he continually wrestled with the heart-wrenching tension between filming and helping those who were suffering.

“I made a decision when I started working as a conflict journalist in Ukraine in 2014. I witnessed so many tragedies, I had to come up with kind of a rule on what to do in these kinds of situations,” he said. “I promised myself that whenever I see someone needing help and if there is someone else who can help, then I'll keep filming. If not, I would try to help.”

Like most journalists, he says he’s always been skeptical about the power of information to create change.

“All conflict journalists want to make a report or an image that will stop the war, and we feel guilty that we can’t do more,” he said. “But I’ve come to realize this work does matter. This work can change people's lives and save people's lives.”

Called a “harrowingly immediate look at a city under siege” by Variety, “haunting and riveting“ by The Los Angeles Times and “one of the most important movies of Sundance“ by, 20 Days in Mariupol has already made an impact on audiences following its Sundance premiere.

Follow FRONTLINE to learn more about when this acclaimed documentary will be available to watch on PBS. Learn more about Chernov and the film here.