A number of police officers have been rushed to hospital following a violent altercation with protesters in Philadelphia.
One 56-year-old female sergeant was struck by a truck and broke her leg.
Tensions in the city have reached a boiling point after the fatal shooting of a 27-year-old black man on Monday.
Walter Wallace Jr was waving a knife around when he was shot, police have said.
Residents protesting police violence gathered outside the local police precinct in the aftermath, smashing windows and spray-painting the building's walls.
Now there are reports that "at least 12" officers have been hospitalised with various injuries, including the sergeant who was struck by the black pick-up truck.
Footage of the incident has circulated online, showing a man standing through the roof of the vehicle.
A witness can be heard saying: "Oh my god. He hit a cop."
A video of Monday's fatal shooting shows Mr Wallace walking around a car into the street toward two police officers pointing their guns at him.
Several shots can be heard before Mr Wallace is then seen lying on the ground. He was hit in the shoulder and chest and was taken to hospital where he died.
The AP quotes police spokesperson Tanya Little as saying the officers had ordered Mr Wallace to drop the knife, but he "advanced towards" them instead.
She said one of the officers "scooped up" the knife after the shooting.
Witness Maurice Holoway told CBS Philly he and several others had attempted to get Mr Wallace to drop the knife but were unsuccessful.
He said if police felt they had to shoot the man, they should have hit him in the leg "or not shoot him at all."
Philadelphia Mayor Jim Kenney said the shooting raised "difficult questions that must be answered".
An investigation into the incident has been opened.
It's the latest fatal police shooting of a black American to spark intense protest in recent months.