The FBI has been asked to join the Texas Rangers in an investigation into how a routine traffic stop led to the arrest of an African-American activist - and how three days later she was found hanging dead her in cell.

The investigation comes after the sudden death of 28-year-old Sandra Bland. Request for FBI assistance came from the Texas Department of Public Safety.

Bland, an activist with the Black Lives Matter movement, was found hanging in her jail cell Monday morning. The Waller County Sheriff’s Office said she was “not breathing from what appears to be self-inflicted asphyxiation.” Although CPR was immediately performed, Bland was pronounced dead shortly after.

Bland was arrested after she had been pulled over by police during a routine traffic stop on Friday, July 10 in Prairie View, Texas, about 60 miles northwest of Houston.

Police said she failed to signal when changing lanes. During the stop, police said Bland became argumentative, uncooperative and combative. They also claimed she was arrested because she had assaulted a public servant.

However, a bystander’s video from a portion of the arrest shows no confrontation. Instead it shows two police officers pinning Bland to the ground, where she is heard arguing with them.

“You just slammed my head into the ground. Do you not even care about that? I can’t even hear!” Bland said in the video.

The video shows one officer leaving her to go to his police car, and telling the bystander to stop filming, which he does not. Then the video shows the officer returning to Bland, handcuffing her, and the two officers picking her up and dragging her to the police vehicle.

Bland was kept in Waller County jail over the weekend, but on Monday, July 13 she was discovered hanging from the ceiling of her cell by a noose made from a trash bag.

Waller County Sheriff Glenn Smith said video from the hall shows no one had gone into Bland’s cell before her death.

Family and friends of Bland said they are stunned by the news and that she would never hurt herself. They said she had been in good spirits and looking forward to starting a new job in the state. Bland was also known as a passionate social justice activist with the Black Lives Matter movement. The Bland family, who live in Chicago, where Sandra was from, have retained legal counsel to investigate her death. They are confident that she was murdered.

“How did a traffic violation end in death? What happened in that jail?” Bland’s friend LaNitra Dean told The Huffington Post. “What happened in that jail? That’s on everyone’s mind.”

Texas bondsman says Sandra Bland tried to bail herself out before her death http://t.co/y4OaKvaL1lpic.twitter.com/eUOP33mr9I — Mic (@micnews) July 18, 2015

Even Waller County District Attorney Elton Mathis said he thought the circumstances of her death were strange.

“I will admit it is strange someone who had everything going for her would have taken her own life,” he told NBC affiliate KPRC in Houston. “That’s why it’s very important a thorough investigation is done and that we get a good picture of what Ms. Bland was going through the last four or five days of her life.”

“If there is something nefarious, or if there was some foul play involved, we’ll get to the bottom of that,” he said.

According to the Guardian, Sheriff Smith has a checkered history and is known for being a racist. He was suspended for two weeks without pay in 2007 when serving as Chief of Police in Hempstead, and ordered to take anger management classes. Smith was also placed on a six-month probation after returning to work, after it was learned he had “manhandled a black suspect” and exhibited “racist behavior.”

In 2008, the Houston Chronicle reported that Smith was fired as police chief after more accusations had surfaced, including accounts of young African-Americans being humiliatingly strip-searched. Later that year, Smith was elected as Waller County’s sheriff. He was re-elected in 2012, the same month James Howell, a 29-year old inmate, was found hanging in his cell at the Waller County jail in an apparent suicide.