US authorities are investigating the death of an African-American woman, who died in a Texas jail cell last week, as a murder case, according to a district attorney.

Sandra Bland, 28, was found hanged in a prison cell at the Waller County jail in Texas on July 13 three days after she was arrested for failing to signal while changing lanes.

On Monday, Texas authorities released a three-hour video taken from outside a jail during the last minutes of Sandra Bland’s life.

They say that the camera is motion-activated and stops recording after 15 seconds without detectable movement, indicating that there is more than nine minutes of time unrecorded.

"It is very much too early to make any kind of determination that this was a suicide or a murder because the investigations are not complete," Waller County District Attorney Elton Mathis told reporters on Monday. "This is being treated like a murder investigation."

"There are too many questions that still need to be resolved,” he said, adding the case would go to a grand jury.

Meanwhile, many people have put into question the police account of Bland's death as those who know her say she was not suicidal.

Bland, who lived in the Chicago suburb of Naperville, Illinois, was in Texas because she was taking a job as a student ambassador to the alumni association at Prairie View A&M University.

According to the Texas Department of Public Safety, whose trooper pulled over Bland, Bland became "argumentative and uncooperative" and was jailed on a charge of assaulting a public servant.

The dash cam video of Bland's arrest is expected to be released Tuesday, according to authorities.

The Texas department of public safety has said the officer, Brian Encinia, had been reassigned to desk duties for “violations of the department’s procedures regarding traffic stops and the department’s courtesy policy.”

The death of the black woman has sparked national outrage and protests amid harsh criticism over police brutality against African-Americans with white officers killing unarmed black men several times during the past months.