At about 7 a.m. Monday, Sandra Bland got her breakfast in the Waller County Jail in Texas. The 28-year-old woman was being held there after a routine traffic stop the previous Friday led to a charge of assault on a public servant.

About two hours later, Bland was dead. The official cause: suicide by hanging.

Now, many of her friends and relatives, along with a growing group of social-justice activists, are questioning how authorities say Bland died — and the circumstances surrounding the arrest that put her behind bars three days earlier.

“The Waller County Jail is trying to rule her death a suicide and Sandy would not have taken her own life,” one friend told the ABC affiliate in Chicago. “Sandy was strong. Strong mentally and spiritually.”

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At a news conference in Chicago, Bland’s sister told reporters on Thursday that she couldn’t believe Bland would take her own life.

“Based on the Sandy that I knew, that’s unfathomable to me,” Sharon Cooper said, according to the Chicago Tribune

The Waller County Sheriff’s Office said in a statement that an unnamed female inmate wasn’t breathing when she was found in her cell Monday morning, at about 9 a.m.

“CPR was immediately started and Waller County EMS notified,” read the statement, sent to The Post on Thursday. “She was pronounced deceased a short time later.”

The sheriff’s office didn’t identify the deceased woman because, it said, investigators were still contacting relatives. But Thursday afternoon, Waller County District Attorney Elton Mathis named Bland and said his office “joins her family in grieving this tragic loss of life.”

An autopsy classified the death as suicide by hanging, Harris County Institute of Forensic Sciences spokeswoman Tricia Bentley told The Post, and the sheriff’s office statement said it appeared to be from “self-inflicted asphyxiation.”

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“The family of Sandra Bland is confident that she was killed and did not commit suicide,” Bland’s family said in a statement sent to the Tribune by the law firm they hired. “The family has retained counsel to investigate Sandy’s death.”

At the press conference, another of Bland’s sisters said that the two had a telephone conversation after Bland was taken into custody. Shante Needham said Bland was “very aggravated,” and thought she had broken her arm, according to the AP.

The Texas Rangers, an investigative arm of the state’s Department of Public Safety, are investigating the death. Additionally, the Department of Public Safety said it has asked for the Federal Bureau of Investigation’s assistance.

“At this time, the joint investigation by the Texas Rangers and the FBI is ongoing,” the release stated.

Shauna Dunlap, a spokeswoman for the FBI’s office in Houston, told the Houston Chronicle in an email that the agency would be “monitoring the local investigation until it is complete.”

“Once the local process takes its course, the FBI reviews all of the evidence and if warranted could pursue a federal investigation,” she wrote.

Mathis, the district attorney, told the Chicago Tribune that Bland was “down from Illinois” to start a new job. ABC7 reported that she was expected to start work at Prairie View A&M University, her alma mater. An email sent to the university was not immediately returned.

“She was a very, very accomplished young lady,” the Rev. James Miller, pastor at DuPage African Methodist Episcopal Church in Lisle, Ill., told the Chicago Tribune. “I just hope that the investigation is very comprehensive.”

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Miller told the newspaper that Bland and her family had been members of the church for about 18 years. The Chicago Tribune reported that she was from Naperville, a suburb of Chicago.

“Any time somebody dies, it’s a tragic deal,” Waller County Sheriff Glenn Smith told the Chicago Tribune. “That’s exactly what this appeared to be.”

After word of Bland’s death spread, hashtags including #JusticeForSandy and#WhatHappenedToSandraBland sprung up on social media, and an online petition calling for a Department of Justice investigation collected more than 15,000 signatures by early Thursday afternoon. On Thursday, Bland’s family said they didn’t want her death politicized, the Chicago Tribune reported.

In his Facebook statement, Mathis, the district attorney, said his office “is actively consulting with and monitoring the investigation being conducted by the Texas Rangers into Ms. Bland’s death. 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.”

He added: “Please allow us to do our jobs, and rest assured that Ms. Bland’s death is receiving the scrutiny it deserves.”

Bland’s arrest and death come amidst a larger national conversation about the use of force in law enforcement and police tactics.

One year ago, Eric Garner died after being placed in a police chokehold. His family recently agreed to a $5.9 million settlement with New York City. Last August, protests broke out after the death of 18-year-old Michael Brown, who was fatally shot by police, in Ferguson, Mo.

There were protests in Cleveland following the fatal police shooting of 12-year-old Tamir Rice in November. In April, riots broke out in Baltimore after a man named Freddie Gray died of a severe spinal injury following an arrest.

An ongoing Washington Post investigation has found that police officers have shot and killed at least 500 people in the United States this year, including high-profile incidents in North Charleston, S.C., and Tulsa, Okla.

Video of a portion of Bland’s arrest, obtained by Chicago’s ABC affiliate, appears to show her down on the ground, held by an officer. She appears to be vocally protesting the circumstances of her arrest during the video, though the recording doesn’t clearly pick up everything she says.

The footage starts after Bland has already been restrained on the ground by an officer.

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

Bland thanked the person filming the arrest as she was being led toward a patrol car, after which the video concludes.