Why was Sandra Bland arrested in the first place?

On Tuesday, Texas authorities are preparing to release dashboard camera footage of the traffic stop and arrest of Sandra Bland, an African-American woman who died in police custody three days after a routine traffic stop.

The police dash-cam footage may help answer questions surrounding her arrest on a rural Texas road on July 10, a seemingly routine stop that quickly escalated, ultimately concluding in Ms. Bland's arrest, jailing, and death.

Bland's death has sparked fresh tension over the treatment of African Americans in police custody, which became a national issue after a series of high-profile deaths including those of Freddie Gray, an unarmed black teen in Baltimore; Eric Garner, an unarmed black man in Staten Island; and Michael Brown, an unarmed black teen in Ferguson, Mo.

Among the questions Bland supporters are asking: Why was she stopped in the first place? Why was she asked to get out of her car? And why was Bland arrested?

As of Tuesday morning, ahead of the public release of the dash-cam video, this is what we know so far:

Bland, originally from suburban Illinois, was pulled over Friday, July 10, after filling out paperwork for a new job in student outreach at Prairie View A&M University, where she was supposed to start work the following week.

Bland was stopped for failing to use her turn signal while changing lanes. According to the family's attorney, Cannon Lambert, Bland expressed in the dash-cam video viewed by legal counsel that she changed lanes to get out of the way of the police car coming up behind her.

While it may appear to be a minor offense committed by thousands of US motorists every day, under Texas transportation code, failing to signal intent to change lanes is a criminal offense. Depending on the circumstances, the offense can result in fines and even jail time.

But that's not why Bland was arrested. After a trooper ran Bland's license and insurance, the situation escalated, and that's where the accounts diverge.

Authorities say Bland “became argumentative and uncooperative.” Elton Mathis, the district attorney in Waller County where Bland was stopped, saw the dash-cam footage and said Bland “was very combative” and that she “was not a model person that was stopped at a traffic stop.”

Trooper Erik Burse, a spokesman for the Texas Department of Public Safety, told the Chicago Tribune that an officer was about to issue Bland a written warning when she kicked the officer and was then arrested on a charge of assaulting a public servant.

Mr. Lambert, the Blands' attorney, who also saw the dash-cam video, tells a different story. According to his account, after checking Bland's paperwork, a trooper asked Bland to put out her cigarette.

Bland, who seemed irritated at having been pulled over in the first place, responded, 'Why do I have to put out a cigarette when I'm in my own car?'" Lambert told NBC News. "And that seemed to irritate him to the point where he said, 'Get out of the car.'"

Bland "wasn't comfortable getting out of the car," Lambert continued, so he "looked to force her to get out of the car by way of opening the door and started demanding that she do."

When the trooper pulled out a Taser and pointed it at Bland, she got out of the car.

While it's unclear exactly what happened next, a cellphone video of the arrest that has surfaced online shows Bland on the ground with a trooper on top of her. She can be heard shouting, "You just slammed my head into the ground. Do you not even care about that? I can't even hear."

In a statement issued after Ms. Bland’s death, state officials said a preliminary review of the traffic stop found that some of the department’s procedures were not followed, according to the New York Times. 

Bland ended up in the Waller County Jail, where, three days later, she was found hanging in her cell.

Her death was ruled a suicide, but Bland's friends and family say Bland would not have harmed herself. Prosecutors are now treating her death like a murder investigation and looking into possible foul play.

Details surrounding Bland's arrest and death are still emerging, but as Think Progress notes, routine traffic stops often escalate when they involve a person of color.

Police are 31 percent more likely to pull over a black driver than a white driver, according to federal statistics. Black drivers are also more likely to be pulled over for minor offenses like a broken tail light or failure to signal a turn. In some situations, they are not even given a reason at all for the stop.

That's why Bland's death, like those of Gray, Garner, and Brown, is raising anew questions about police treatment of African Americans.