Sandra Bland was stopped by Texas police for a lane change and ended up dead in a jail cell

Grieving friends and family of Sandra Bland want to know how the 28-year-old wound up dead in a Texas jail cell, and they don't believe the story they're being told by Waller County authorities.

"The Waller County Jail is trying to rule her death a suicide and Sandy would not have taken her own life," LaNitra Dean, a long-time friend, told ABC 7 Chicago. "Sandy was strong. Strong mentally and spiritually."

Bland, who was outspoken about racism and police brutality, had just gotten a new job at her alma matter, Texas Prairie View A&M, and was on her way from the Chicago suburb of Naperville to her new home near Houston when she was stopped by police for failing to signal during a lane change on Friday afternoon, the Chicago Tribune reports.

The simple traffic stop escalated, according to video of the arrest that has emerged on YouTube. The video shows officers on top of Bland as she cries out that they had slammed her head into the ground and were being too rough with her. An officer is seen telling the person shooting the video to leave. The recording does not show the beginning of the encounter.

"You just slammed my head into the ground," Bland yells as she is being arrested. "Do you not even care about that? I can't even hear!"

She was arrested for assaulting a police officer, according to news reports. Waller County Sheriff Glenn Smith claimed she "had been combative on the side of the road."

On Monday morning, jailers found her dead, and officials attributed it to self-inflicted asphyxiation.

"CPR was immediately started and Waller County EMS notified. She was pronounced deceased a short time later. In order to maintain the integrity of the investigation, the Texas Rangers were immediately notified to conduct the death investigation," the sheriff's department said in a statement.

The Chicago Tribune reports her death was ruled a suicide through hanging.

Authorities told reporters there are no cameras in her jail cell and no footage of her death.

"I do not have any information that would make me think it was anything other than just a suicide," Waller County D.A. Elton Mathis told the news station.

Her friends and family disagree, and so do many others. As of Thursday, #SandraBland was trending on Twitter.

Her family has retained a lawyer and will conduct its own investigation of her death, saying they are "confident" she was killed. The Texas Rangers have been called in and will also investigate the death.

Posts to Sandra Bland's Facebook page seem to indicate an upbeat woman, showing a grinning Bland with her mother, and talk of eating at the Waller County Line restaurant.

She also posted about police violence and racial discrimination. Her chillingly-prophetic cover photo is a cartoon depicting a bloodied, beaten black man handcuffed in a police car, seated next to an unscathed, hamburger-eating, Confederate flag-wearing Dylann Roof.

“Sandy became very proactive in using social media to bring awareness to police brutality,” a friend wrote in an email to NBC 5. “Family and friends have no doubt that foul play was involved in this alleged suicide. She was a very outspoken individual who loved life, went to Church and loved her family.”

According to the Daily Kos, Waller County Sheriff Glenn Smith was suspended, then fired in 2007 from a previous job as Hempstead police chief over documented cases of racism.

Watch the report from ABC News, which includes video of Bland's arrest below: