A Texas state trooper tried to pull black motorist Sandra Bland out of her car, then drew his stun gun and threatened her after she refused to follow his orders during a traffic stop.
A police video shows what happened during the encounter, days before Ms Bland was found dead in a jail cell, in a case that has caused her family and supporters to dispute that she hanged herself, as authorities have said.
In the footage, the incident swiftly escalates into a shouting confrontation as officer Brian Encinia tries to drag the 28-year-old from her vehicle, with him saying at one point "I will light you up", a possible threat to use the stun gun.
The video posted by the Texas Department of Public Safety shows Encinia stopping Ms Bland for failure to signal a lane change.
After he hands her a written warning, he says she seems irritated, and the Illinois woman replies that she had changed lanes to make way for his car.
The conversation turns hostile when Encinia asks Ms Bland to put out her cigarette and she asks why she cannot smoke in her own car.
He then orders her to get out of the vehicle.
She refuses, and he tells her she is under arrest.
Further refusals to get out bring a threat from Encinia to drag her out. He then pulls what appears to be a weapon and says: "I will light you up."
After Ms Bland finally steps out of the vehicle, the confrontation continues off-camera but is still audible. The two keep yelling at each other as Encinia tries to put her in handcuffs.
He has since been placed on administrative leave.
Ms Bland was taken to the Waller County Jail about 60 miles from Houston on July 10 and found dead on July 13. A Texas Rangers investigation into her death is being supervised by the FBI.
Although a medical examiner has ruled the death a suicide, supporters insist she was upbeat and looking forward to a new job.