Sandra Bland’s injuries were consistent with suicide, a post-mortem examination has found.
The 28-year-old African-American woman was found dead in a Texas prison cell on July 13, three days after her arrest.
Marks on Sandra Bland’s neck suggested she killed herself, said Waller County prosecutor Warren Diepraam.
About 30 cuts on her left wrist were likely to have been self-inflicted some weeks before, he said.
A video of the arrest shows a police officer pulling over Sandra Bland for not signaling when changing lanes.
After issuing a ticket, a confrontation follows when Sandra Bland refuses to stub out her cigarette or get out of the car.
As the action moved off camera, she can be heard saying the police officer slammed her head to the ground.
Prosecutor Warren Diepraam said there were only some lacerations and abrasions consistent with being handcuffed.
There were no “defensive injuries” usually found in a murder, he added.
Jail records show that Sandra Bland told a member of staff she had attempted suicide in the past.
But guards assessed her and determined she was not suicidal when she was booked into the county jail.
State officials and the FBI are investigating her death.
It also emerged that Sandra Bland left a voicemail to a friend from jail to say she was “at a loss” over her arrest.
In a voicemail obtained by ABC, Sandra Bland said she could not understand why a traffic violation had led to her detention.
In the message to a friend Sandra Bland said: “I’m still just at a loss for words, honestly, about this whole process. How did switching lanes with no signal turn into all of this?”