Last Updated Jul 22, 2015 4:00 AM EDT
Dashcam video shows the moment 28-year-old Sandra Bland was stopped by Texas trooper Brian Encinia for failing to signal a lane change.
In the July 10 encounter, Encinia asks Bland to put out her cigarette. Bland refuses and the trooper demands that she get out of the car, opening the door with his taser drawn.
The two drift out of camera view. Encinia claims that's when she assaulted him. A witness captured part of the arrest on his cell phone.
"You slammed my head into the ground, do you even care about that," Bland is heard saying in the video. "I can't even hear."
"You question why in a routine traffic stop when the officer was going to write a warning, it was even necessary for her to get out, just give her the warning and let her go on her way," said Cannon Lambert, the Bland family attorney.
Three days after her arrest, the Naperville, Illinois native was found dead in a Waller County jail cell. The medical examiner has ruled it a suicide "by self-induced asphyxiation." Bland's sister, Sharon Cooper doesn't believe it.
"My sister was someone who was full of life, who had so much to live for," she said. "That's why I think it's unfathomable, based on the person I know."
The death is bring investigated by the FBI and Texas Rangers.
CBS Houston affiliate KHOU-TV reports family members attended a memorial service for Sandra on the campus of Prairie View A&M Tuesday night. Hundreds gathered in the intimate chapel on the campus of Bland's alma mater.
Cooper read scripture while her mother, Geneva Reed-Veal, spoke about the ongoing investigation into her daughter's death. "There is not anywhere that I can see that my baby took her own life," Reed-Veal said.
Also Tuesday, the director of the Texas Department of Public Safety said Encinia was in the wrong, according to KHOU..
"A DPS state trooper has an obligation to exhibit professionalism and be courteous throughout the entire contact and that wasn't the case here," Steven McCraw said.
Encinia has been put on desk duty for violating departmental procedures.
Late Monday, the Waller County Sheriff released surveillance video of the hallway outside Bland's cell as well as a timeline. At 7:05 a.m., Bland tells a deputy during a routine check, "I'm fine." At 7:55 a.m. Bland asks a deputy if she could make a phone call. No one is seen coming or going from her cell. At 8:58 police say a deputy found her unresponsive with a plastic trash bag around her neck, hanging from a privacy partition.
The surveillance video shows authorities rushing into her cell to try and revive her.
Waller County District Attorney Elton Mathis said the case was being treated like a murder investigation and he plans on sending it to a grand jury as early as next month.