The circumstances surrounding the death of a woman in a Texas county jail are under investigation after Sandra Bland’s friends and relatives expressed skepticism that the person they knew would, as authorities have said, take her own life.

As the investigation continues, here’s what we know about the case.

Who was Sandra Bland?

Sandra Bland was a 28-year-old African American woman who was pulled over in Texas last week.

According to reports, Bland was set to start a new job at her alma mater, Prairie View A&M University, a historically black college located in Waller County. She had returned to Texas from Illinois, where she grew up.

According to the Texas Department of Public Safety, on July 10, a trooper stopped the Hyundai that Bland was driving in Waller County because she didn’t signal to change lanes. Bland “became argumentative and uncooperative” during the traffic stop, the department said in a news release, and she was taken into custody.

Bland was charged with assault on a public servant and taken to the Waller County Jail. A police officer from Prairie View was also at the scene of the arrest, in addition to the DPS trooper, according to the release.

In a preliminary review of Bland’s traffic stop, the Texas Department of Public Safety announced Friday that “we have identified violations of the department’s procedures regarding traffic stops and the department’s courtesy policy.” Video footage of the arrest will be made public “as soon as possible” and the officer involved in the stop has been placed on administrative duties.

A portion of Bland’s arrest was caught on video, according to Chicago’s ABC affiliate. That footage shows a woman on the ground, held down by an officer.

“We have little reason to believe it’s not her,” a Bland family attorney said of video, according to the Chicago Tribune. “We have every reason to believe that it was.”

The woman in the video appears to be vocally protesting, though the recording doesn’t catch everything she says.

“You just slammed my head into the ground, do you not even care about that?” Bland can be heard saying during the encounter. “I can’t even hear.”

Three days later, she was dead.

The Waller County, Tex., sheriff’s department released video showing the area where Sandra Bland was found hanged to death in her prison cell. The FBI and the Texas Rangers are investigating her death. (Reuters)

How did Sandra Bland die?

Sandra Bland died Monday morning in her cell, about two hours after she got her breakfast in the Waller County Jail.

An autopsy classified Bland’s death as suicide by hanging, Harris County Institute of Forensic Sciences spokeswoman Tricia Bentley told The Post. At a news conference Thursday, authorities gave more details of her death, saying that it appeared she had hanged herself with a trash bag.

“Trash bags are a day-to-day item in the cells,” Waller County Sheriff R. Glenn Smith told reporters, according to the Houston Chronicle.

News of the suicide was met with skepticism by those who knew Bland, and hashtags such as #JusticeForSandy and #WhatHappenedToSandraBland soon sprang up on social media as those who didn’t know her learned of her death.

“When you think through the circumstances that have been shared with us to this point, it is unimaginable and difficult for us to wrap our minds around,” her sister, Sharon Cooper, said at a news conference, according to the Chicago Tribune.

Why are people skeptical?

Bland’s death came amid growing outrage over police interactions with African Americans.

She died nearly a year to the day after Eric Garner was killed after being placed in a police chokehold. Her death also followed those of 18-year-old Michael Brown, who was fatally shot by a police officer last August in Ferguson, Mo., and 12-year-old Tamir Rice, who was killed by Cleveland police in November, and Freddie Gray, whose death of a severe spinal injury after an arrest set off riots in Baltimore. There were high-profile incidents in North Charleston, S.C., and Tulsa, Okla., as well.

Here’s how Bland’s acquaintances and family members have described her:

• “She was a very, very accomplished young lady” — the Rev. James Miller, pastor at DuPage African Methodist Episcopal Church in Lisle, Ill. (Chicago Tribune)

• A “warm, affectionate, outspoken woman” — LaNitra Dean, friend of Bland (ABC7)

• “To know Sandy was to love her” — Cooper, Bland’s sister (Houston Chronicle)

“It doesn’t make sense that this young lady who appeared to be healthy would have reason to commit suicide,” James Douglas, a law professor who also serves as president of the Houston branch of the NAACP told the Tribune. “Supposedly she had some damage when she was thrown to the pavement. She may have suffered a concussion or internal brain damage. That’s what we want to make sure.”

However, the Associated Press reported that Bland might have been depressed earlier this year. The AP cited a video posted on her Facebook page, in which Bland says she’s had “a little bit of depression.”

What else do we know?

As the investigation into the circumstances surrounding Bland’s death continues, Waller County’s history of racial tension has come under scrutiny.

The county’s elected sheriff, Glenn R. Smith, was fired from his job as a city police chief within the county a few years ago. Smith has said that he believes his firing was politically motivated and connected to his plans to run for sheriff. However, the council made its decision to suspend — and eventually fire — Smith after hearing complaints of racism from the community, the Houston Chronicle reported at the time.

Waller County has a total population of about 47,000, according to the census. About a quarter of that population is African American.

A local activist named Hershel Smith told the paper that community members were outraged by a drug raid conducted at the wrong house in town. The activist also said that the city’s police force had forced young African American men to undergo a strip search in public.

On Thursday, the sheriff reiterated that he believes he was fired from his previous job for political, and not racial, reasons. “I am not a racist,” Smith said at a news conference. He added: “Black lives matter to Glenn Smith.”

The Waller County Sheriff’s Office runs the jail in which Bland was held at the time of her death, although we should reiterate that she was arrested by a trooper from the Texas Department of Public Safety.

Bland’s death also resurfaced scrutiny of an earlier death in the Waller County Jail: In 2012, a 29-year-old inmate named James Harper Howell IV was found hanging from a bed sheet in the jail. Howell, who was white, was facing charges of assault on a public servant, evading arrest/detention with a vehicle and possession of marijuana, according to the Houston Chronicle.

His death was investigated by the Texas Rangers.

What happens now?

Bland’s death is under investigation.

The Texas Department of Public Safety said in its statement that it asked for FBI assistance, and a spokeswoman for the agency’s office in Houston told the Chronicle that federal authorities there are “monitoring the local investigation until it is complete.”

“The D.A.’s \Office is actively consulting with and monitoring the investigation being conducted by the Texas Rangers into Ms. Bland’s death,” Waller County District Attorney Elton Mathis wrote in a Facebook post. “Once the investigation is complete the matter will be turned over to a Waller County grand jury for any further proceedings deemed appropriate by them.”

[This post has been updated. Elahe Izadi contributed.]