Police say they found the 28-year-old dead last week after she hanged herself with a plastic bag inside the Waller County Jail, where she was incarcerated after allegedly assaulting an officer during a July 10 traffic stop.
The Texas Rangers and the FBI are investigating her death.
"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. "This is being treated like a murder investigation."
He said the case would go to a grand jury.
"There are too many questions that still need to be resolved. Ms. Bland's family does make valid points that she did have a lot of things going on in her life that were good," Mathis said.
Bland lived 1,000 miles away in the Chicago suburb of Naperville, Illinois, but was in Texas because she was taking a job as a student ambassador to the alumni association at Prairie View A&M University. She graduated from the historically black school in 2009.
The Texas Department of Public Safety, whose trooper pulled over Bland for allegedly failing to use her signal while changing lanes, has issued a statement saying Bland became "argumentative and uncooperative" and was arrested on a charge of assaulting a public servant.
According to a sheriff's office statement, she was found "in her cell not breathing from what appears to be self-inflicted asphyxiation." Bland received CPR, and an ambulance was called, but she was pronounced dead a short time later.
Many people have questioned the police account of Bland's death. Those who know her say she was anything but suicidal.
"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."
There were no cameras inside Bland's cell but cameras in the hallway showed no one entering or leaving before her body was discovered, Mathis has said. Police dashcam video is expected to be released on Tuesday.