Family and friends sought more details Thursday about the death of a black woman who authorities say hanged herself in a Texas jail after her arrest for allegedly kicking an officer following a traffic stop, saying the 28-year-old gave no indication she was in such an emotional state that she would kill herself.

However, Sandra Bland had posted a video to her Facebook page in March acknowledging she was suffering from "a little bit of depression as well as PTSD," or post-traumatic stress disorder.

The Texas Rangers and the FBI are investigating the circumstances surrounding Bland's death, and a prosecutor said he planned to present the state findings to a grand jury. Bland was found dead Monday morning in a Waller County jail cell in Hempstead, Texas, about 60 miles northwest of Houston.

"Based on the Sandy that I knew, that's unfathomable to me," Bland's sister, Sharon Cooper, said at a news conference in Chicago.

Bland, who was from the Chicago suburb of Naperville, was moving to Texas to work at Prairie View A&M University, the historically black college from which she graduated in 2009. She was arrested last Friday in Prairie View.

Her death comes amid increased national scrutiny of police after a series of high-profile cases in which blacks have been killed by officers. Social media posts have questioned the official account of her death.

Many posts have featured #SandySpeaks, which refers to the hashtag Bland had used in monologues she posted on Facebook. Among other things, she talked about police brutality and what she said was a calling from God to speak out against racism and injustice.

"I'm here to change history," she said in one video. "If we want a change we can really truly make it happen."

In one of the videos, she addresses her depression. She apologized in the brief March 1 message for not posting any videos for two weeks, saying she's been having "depressed moments." She doesn't explain the cause of the PTSD.

At a news conference Thursday, Waller County District Attorney Elton Mathis said it would be up to a grand jury to decide the importance of that video.

An attorney hired by Bland's family, Cannon Lambert, said some relatives believe Bland was killed and the family wants more information from investigators.

"This family is really looking to understand what happened," Lambert said. "We don't understand this. It doesn't make sense."

He did not return messages left later asking about the video in which Bland talked about being depressed.

Mathis said an autopsy found Bland died by asphyxiation and that she used a plastic bag to hang herself from a partition in her cell. He also said that although jail video didn't show what went on in Bland's cell, it showed no one went in or out of it from the time she was placed there until a jailer found her unconscious.

Sheriff Glenn Smith said jailers had used an intercom to check on Bland less than an hour before she was found dead.

The Texas Commission of Jail Standards cited the Waller County jail three years ago for improperly monitoring prisoners. The state agency found the jail was not checking all inmates at least once an hour, as required by law. It inspected the jail after a man hanged himself with a bedsheet in November 2012.

Bland's sister, Shante Needham, said Bland had called her from jail Saturday afternoon, telling her that she'd been arrested, but didn't know why. She also said an officer had placed his knee in her back and she thought her arm had been broken.

"She was very aggravated. She seemed to be in pain. She really felt that her arm had been fractured," Needham said, holding back tears. "I told her I would work on getting her out."

The Texas Department of Public Safety said Bland "became argumentative and uncooperative" after a trooper stopped her for failure to signal a lane change. She was then arrested for assault on a public servant. The department said paramedics were called to the scene, but Bland refused a medical evaluation.

Department spokesman Erik Burse told the Chicago Tribune that Bland was outside the car and about to be issued a written warning when she kicked the officer. Lambert said it would have been out of character for her to kick the officer if unprovoked.

A cellphone video posted online purporting to show part of Bland's arrest shows an officer pinning a woman to the ground with one knee. At one point the woman can be heard yelling that she can't "feel my arm."

"You just slammed my head into the ground," she says. "Do you not even care about that?"

The Associated Press could not independently verify that the video is of Bland's arrest, but the images are consistent with information the family gleaned from Bland's jailhouse phone call.

Lambert and Needham said they'd been working to get the money for bail when they learned of Bland's death.