Surveillance video of the 2014 incident shows a 15-year-old girl pedaling a bike through a mall parking lot in Washington state.
An off-duty officer in an SUV pulls up from behind and stops the girl, who was with her brother. The girl, Monique Tillman, asked the officer why the pair had been stopped, according to an amended lawsuit complaint filed in Pierce County Superior Court.
The Tacoma police officer, identified in the lawsuit as Jared Williams, told Tillman and her brother, Eric Branch, that they were causing a disturbance, the lawsuit alleges - something that Tillman disputed.
Tillman, who is black, told authorities that she thought she was "being harassed because of the colour of her skin," the lawsuit states. After a back-and-forth, Tillman claims she tried to pedal away.
Instead, the lawsuit states, the officer "erupted" and "began brutalising" the teen. He "tossed Plaintiff Tillman around like a child's doll, slamming her into parked vehicles, forcefully shoving his hand and forearm into her chest, grabbing her by the hair and body slamming her into the pavement," the complaint claims.
When Tillman was on the ground, the officer used a stun gun on her, the complaint states. Her brother, "horrified" by the scene, tried to help. But according to the lawsuit, he was threatened with the stun gun, too.
In the wake of the incident, Tillman has suffered nightmares, fears incarceration and distrusts police officers, the lawsuits states. According to the lawsuit, Williams was "working as a Tacoma police officer and in an off-duty capacity" in a private security job at the time the incident occurred.
"She says it best herself, and that is she doesn't feel secure," her attorney, Vito de la Cruz, said in a phone interview. "She is frightened of police officers and doesn't feel like she is safe."
Tillman was arrested after the May 2014 incident but the charges she faced were dismissed by a judge, according to the Seattle Times, which reports that the lawsuit was initially filed earlier this year. The lawsuit - in which Tillman claims her civil rights were violated during the encounter with police and security guards - has now been amended and expanded.
The Times reports that this happened "after a spokeswoman for Tacoma police told reporters that though Williams was off duty and working as a security guard, he still had police authority to make the arrest."
According to a news release from de la Cruz, the amended complaint now includes the Tacoma police department, its chief, and two other defendants, another officer and a security guard.
"Given that Officer Williams' conduct was committed under colour of law as a police officer for the Tacoma Police Department, the Plaintiffs have alleged federal and state civil rights violations in their amended complaint," the release states. "At the core of the complaint is the excessive force used and illegal arrest made by Tacoma Police Officer Williams and others against Ms Tillman and her brother Eric Branch."
The lawsuit, in which Branch is also a plaintiff, alleges that officials "routinely approve or ratify abusive, excessive, and unnecessary uses of force" and fail to take action against officers who go too far.
Authorities allow officers who "abuse citizens and violate their rights" to remain employed, and "routinely charge arrestees with acts of resistance or violence when the officers have used violence against the arrestee, regardless of merit," the lawsuit states.
"I think it's important to stress that police officers are there to protect and serve the community," de la Cruz said. "And in situations such as this, when excessive force was clearly used, the video speaks for itself, they must be held accountable and policies must change. For too long, the African American community and other communities of colour have felt that they're not protected when they're out and have police encounters.
"These two children were riding their bikes and that was all they were doing. And they're African American, and that seems to have been the reason why they were stopped. At least a prime motivator for it."
Williams is white, according to the Associated Press.
The lawsuit comes at a time of increased tensions between law enforcement officers and the communities they serve. Across the country, many are debating the use of force, police tactics and the interactions between law enforcement and black citizens.
Earlier this summer, fatal police shootings in Louisiana and Minnesota stirred continued unrest, while the fatal shootings of officers in Dallas stoked fears about the dangers faced by those who work in law enforcement.
More recently, NFL quarterback Colin Kaepernick has protested by taking a knee during the national anthem, an effort to raise awareness of racial injustice. He has been joined by soccer star Megan Rapinoe, though her act of protest was preempted earlier this week.
A message left with a Tacoma police spokesman was not immediately returned Thursday.