More than 50,000 people have signed an online petition calling for the Justice Department to probe the mysterious death of an African-American woman who died in a Texas jail. Twenty-eight-year-old Sandra Bland was arrested Friday after a traffic stop for failing to use her turn signal while changing lanes. Three days later, on Monday, she was found dead in her jail cell in Waller County, northwest of Houston. Jail officials said her death was a suicide, a claim disputed by her family.

Sharon Cooper, Sandra Bland’s sister: “We want to seek to understand what happened. We don’t know.”

On Thursday, video footage surfaced of her arrest, which appears to show police pushing Sandra Bland to the pavement and restraining her as she asks why she is being treated so roughly and why they slammed her head on the ground. Bland was involved in the Black Lives Matter movement and had posted a series of videos online speaking out against police brutality and racial injustice.

Sandra Bland: “I want the white folks to really understand out there, black people are truly — we’re doing as much as we can. Not all of us, but some of us are really doing as much as we can. And we can’t help but get [bleep] off when we see situations where it’s clear the black life didn’t matter. For those of you questioning why was he running away, well, [bleep], because in the news that we’ve seen as of late, you could stand there, surrender to the cops and still be killed.”