Three years after her mysterious death, the life and legacy of Sandra Bland continues to motivate activists against police brutality. The new HBO documentary Say Her Name: The Death and Life of Sandra Bland explores the circumstances around Bland’s alleged suicide as her family attempts to put together the events of July 13, 2015.

The first trailer for the documentary shows Bland’s family traveling to Waller County, Texas, where Bland died, and discussing the footage shown to them by the police. “We were told: ‘Once you get here, it will be proven, without a doubt. You’ll be able to see that she contributed in her own death,'” Bland’s mother Geneva Reed-Veal says in the trailer. Reed-Veal filed a wrongful death lawsuit after Bland’s death and later reached a settlement of $1.9 million with Texas authorities in 2016.

The footage, as revealed in the trailer, didn’t show Bland’s death. In fact, it didn’t show Bland at all, which left the family with even more questions. “The Texas Rangers, they walked us through footage...of the jail. We were looking, trying to see where Sandy was. I didn’t see her ever,” the family’s attorney Cannon Lambert says.

As videos of daily prison operations play in the trailer, her family reveals that the footage was insufficient in more ways than one. “The video that we viewed when we went down [to Texas], it was only for the morning of Monday, July 13. There are no timestamps, there are no dates. Her cell was all the way at the back corner. She was in cell 95. The way they choose to phrase it is, ‘Where she was did not have cameras,'” Bland’s sister Shavon explains. “I think that’d be strange. Then how are you monitoring your inmates?”

The 28-year-old was arrested on July 10 for failing to signal when changing lanes. Footage of Bland’s arrest surfaced in 2015, showing police officer Brian Encinia forcing Bland out of the car and arresting her aggressively after she refused to put out her cigarette. Encinia has since been fired.

Three days after the arrest, Bland was found hanging in her cell. Her death sparked outrage, protests, and a number of different theories about what really happened. This new film aims to explore never-before-told details about the case and the legal battle that followed Bland’s death.

Say Her Name: The Death and Life of Sandra Bland is set to premiere on April 25 at the 2018 Tribeca Film Festival. It will debut sometime later this year on HBO.