A black woman who authorities say hanged herself in a Texas jail had posted an online video earlier this year saying she was suffering from "a little bit of depression," though family and friends say the 28-year-old gave no indication she was distraught enough to contemplate suicide.

As Sandra Bland's family and friends press for details of what happened, a prosecutor said Thursday that he will present the findings of a Texas Rangers' investigation to a grand jury. The FBI is also investigating the circumstances surrounding Bland's death.

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 on July 10 in Prairie View following a traffic stop, and authorities say she hanged herself Monday morning in a Waller County Jail cell in nearby Hempstead, about 60 miles northwest of Houston.

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

Bland had posted a video to her Facebook page on March 1 in which she said that she was suffering from "a little bit of depression as well as PTSD," or post-traumatic stress disorder. She did not explain the cause of the PTSD.

In a video posted three days later, she elaborated.

"I want you guys to know it's a daily struggle. It's a daily test," she said. "Depression is nothing but the devil. It's a way of mind and it's a way of thinking." She recommended prayer to cut through the fog.

Family members were shocked by her death and do not believe she would commit suicide.

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

Another 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."

Waller County District Attorney Elton 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 on 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.

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Keyser reported from Chicago. Writers Don Babwin and Sara Burnett also contributed to this report from Chicago