HEMPSTEAD, Texas (KRON/AP) – The death of a black woman whose jail cell hanging has been ruled a suicide is now being treated as a murder investigation, according to a Texas prosecutor.

Waller County District Attorney Elton Mathis said Monday that there are many unanswered questions surrounding the death of Sandra Bland, who died three days after a confrontational traffic stop.

A Texas Rangers investigation into her death is being supervised by the FBI, Mathis told a news conference.

Authorities have said the 28-year-old woman from Naperville, Illinois, hanged herself with a plastic garbage bag July 13.

It’s “much too early to make any kind of determination that this was a suicide or a murder because the investigations are not complete,” Mathis said.

He said he’s asked for extensive scientific testing, including for fingerprints and DNA, “so we can figure out and say with certainty what happened in that cell.”

“This investigation is still being treated just as it would be in a murder investigation,” Mathis said, noting that the matter would go to a grand jury.

At the same news conference, Capt. Brian Cantrell, head of the sheriff’s department criminal investigation division, said Bland’s death “was a tragic incident, not one of criminal intent or a criminal act.” But he said he welcomed the investigation.

Although a medical examiner has ruled Bland’s death a suicide, supporters insist she was upbeat and looking forward to a new job at Prairie View A&M University, where she graduated in 2009. Bland’s family and clergy members have called for a Justice Department probe, and an independent autopsy has been ordered.

Bland posted a video to her Facebook page in March, saying she was suffering from “a little bit of depression as well as PTSD,” or post-traumatic stress disorder. Family members have said nothing in her background suggested she was mentally troubled, and at least one friend said she was just venting after a bad day.

Her death at the Waller County jail, about 60 miles northwest of Houston, comes after nearly a year of heighted national scrutiny of police and their dealings with black suspects who have been killed by officers.

The case has resonated on social media, with posts questioning the official account and featuring the hashtags #JusticeForSandy and #WhatHappenedToSandyBland. Others referred to #SandySpeaks, the hashtag Bland used in monologues she posted on Facebook in which she talked about police brutality and what she said was a calling from God to speak out against racism and injustice.

Also on Monday, authorities released a three-hour video from outside Bland’s jail cell and said they expected to release dashcam footage of her arrest Tuesday.

The jail video shows a period of about 90 minutes with no activity in the hallway leading to Bland’s cell. The video does not show the inside of her cell or even her cell door.

It then shows a deputy reacting to what she sees while looking in the cell, triggering a frenzy of activity. An EMT crew arrives with a wheeled stretcher. Deputies and medical personnel are seen coming and going, but a body isn’t visible.

Cantrell said the video was motion-sensitive, indicating if nothing is taking place after a certain amount of time, it turns off. He said the FBI has been given hard drives to determine if there’s been any manipulation.

A guard checked with Bland about two hours before she was found dead and Bland told her, “I’m fine.” About an hour later, Bland asked to make a telephone call and was advised over an intercom that the phone was on a wall in the cell, according to Cantrell. There is no record of her ever making a phone call, he said.

Cantrell declined to describe Bland’s death in detail, though he said she was found with her feet touching the ground. He described the plastic garbage bag used as a ligature by extending his hands about 5 to 6 feet apart.

The bags, he said, had been approved by a jail inspector, but have since been removed from all cells.

Bland’s sister, Shante Needham, has said Bland called her from jail the afternoon after her arrest, telling Needham an officer had placed his knee in her back and she thought her arm had been broken. The Department of Public Safety has said Bland “became argumentative and uncooperative” during the traffic stop. Paramedics were called to the scene but Bland refused a medical evaluation, authorities said.

Mathis said the dashcam video is consistent with information the officer has provided about the traffic stop, though he said it shows only restricted views of the encounter.

Bland “was very combative,” Mathis said. “It was not a model traffic stop … and it was not a model person that was stopped.”

A cellphone video posted online purporting to include 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?”

Public-safety officials have said the trooper who stopped Bland violated traffic-stop procedures and the department’s courtesy policy but have not elaborated. The trooper is on administrative leave pending the outcome of an investigation.