CNN - (CNN) - Sandra Bland was taking a new job in Texas at Prairie View A&M University, the historically black college from which she graduated in 2009.
The prospect of a becoming a student ambassador at her alma mater, however, began to unravel one week ago. Bland was arrested July 10, accused of assaulting an officer after a routine traffic stop more than 1,000 miles from her home in the Chicago suburb of Naperville, Illinois.
Bland, 28, was found dead Monday in a Waller County jail cell in Hempstead, Texas, after authorities said she hanged herself with a plastic trash bag. It is an act those close to her question.
"Based on the Sandy I knew, this is unfathomable to me," Sharon Cooper, one of Bland's sisters, told reporters in Chicago. "People who knew her, truly knew her, the depth of her, that's unfathomable right now."
Demonstrators on Friday marched from the city jail to the courthouse in Hempstead, CNN Chicago affiliate WLS reported.
The Texas Rangers and the FBI are investigating the death, which was discovered days after a Texas Department of Public Safety trooper pulled over Bland for allegedly failing to signal a lane change, authorities said.
"The death of Sandra Bland will not be swept under the rug," Waller County District Attorney Elton Mathis told reporters Friday. "There will be no one who is protected... The truth will come out."
There were no cameras inside Bland's cell but cameras in the hallway outside showed no one entering or leaving before her body was discovered, Mathis said.
The video and police dashcam video will be released publicly, he said.
"We cannot find where anyone goes into the cell from the hallway to do her any harm from the last time she was physically viewed alive," the prosecutor said.
After last week's traffic stop, Blank became "argumentative and uncooperative," according to a public safety department statement. Bland was arrested on a charge of assaulting a public servant, the statement said.
Cell phone video purporting to show part of Bland's arrest was posted online. CNN could not verify the video was of Bland's arrest, but an attorney for her family, Cannon Lambert, said there was "little reason to believe that it's not her."
The video shows an officer using his knee to hold a woman down on the ground. The woman is heard saying that she can't "feel my arm" and that the trooper "slammed my f------ head to the ground."
"Do you not even care about that?" she asked.
In a statement, the Texas Department of Public Safety said the officer involved in Bland's arrest was assigned to administrative duties pending an investigation.
"We have identified violations of the department's procedures regarding traffic stops and the department's courtesy policy," the department said in the statement.
Waller County paramedics were called to the scene of her arrest, but Bland refused a medical evaluation before she was booked, according to the public safety department. Asked about reports that Bland kicked the trooper before her arrest, Lambert said: "It's not characteristic that she would voluntarily and without any sort of provocation strike out at someone."
On Monday morning, Bland was found dead in her cell, according to the Waller County Sheriff's Office.
She was found "in her cell not breathing from what appears to be self-inflicted asphyxiation," a sheriff's office statement said. Bland received CPR, and an ambulance was called, but she was pronounced dead a short time later.
"Any loss of life is a tragic incident and, while the investigation is being conducted by outside agencies, the Waller County Sheriff's Office will continue to observe the daily operations of the jail to always look for improvements and/or preventions of these incidents," the statement said.
Bland's sister, Shante Needham, said her sibling called her from jail Saturday. Needham said her sister told her that a trooper had held her down with his knee on her back before her arrest and that she thought she had fractured her arm.
"She was very aggravated," the sister said. "She seemed to be in pain."
Holding back tears, Needham added: "I told her I would work on getting her out."
Lambert said the family was securing the 10% of Bland's $5,000 bail for her release.
"We don't understand this," the lawyer said of her death. "It doesn't make sense."
Cooper said she can't believe her sister took her own life.
"To know Sandy was to love her," she said. "She was someone who was extremely spontaneous, spunky, outgoing, truly filled with life and joy. So when you think through the circumstances shared with us through this point, this is unimaginable."
But Mathis said investigators were examining online videos posted by Bland in March in which she talked about battling depression and post-traumatic stress disorder. Lambert said members of Bland's family were traveling to Texas.
"This family is really looking to understand what happened," he said at a news conference with relatives.
Mathis said investigators will meet with the family on Monday.
Since her death, posts with the hashtag #JusticeForSandy and #WhatHappenedToSandyBland have gone up on Facebook and Twitter.
"We don't want to see Sandy politicized," Lambert said. "We don't want to see her life politicized, and we don't want to see her death politicized."
He added, "A lot of people are saying that there was foul play. We don't know that there was. We don't know that there wasn't."
Some posts questioning the woman's death use the #SandySpeaks hashtag, which Bland used on Facebook with videos in which she denounced police brutality and racism.
"Being a black person in America is very, very hard," she said in a video posted in April. "At the moment black lives matter. They matter."
CNN's Jareen Imam contributed to this report.
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