Chicago's police superintendent has suggested that an officer's body camera wasn't turned on when he fatally shot a black teen last month because the officer had only received it about a week earlier and wasn't proficient in using it.
But demonstrators who held a march protesting the killing voiced strong suspicions that the camera may have been turned off as part of a cover-up.
At a news conference on Saturday, Superintendent Eddie Johnson discussed nine videos taken from dashcams in police cars and body cameras on other officers involved.
The videos show officers firing repeatedly at a stolen car as it careens down the street away from them.
They also show the officers handcuffing a wounded Paul O'Neal, who was driving the stolen car, after a chaotic foot chase through a residential neighbourhood in the city's south.
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"They had had those cameras maybe about a week ... There's going to be a learning curve," Johnson said of the body cameras.
The cameras were introduced to one police district early last year as part of a pilot project.
They have since been distributed to six other districts and the officer who shot O'Neal had been issued with a camera as part of that rollout, said department spokesman Anthony Guglielmi.
He did not know when officers in the rest of the city's 22 districts would be issued body cameras.
Protesters said on Saturday they did not believe any official explanation for the non-working body camera.
They and the attorney representing the O'Neal family scoffed when a department spokesman said on Friday that the officer's camera may have been deactivated by the force of the air bag when the stolen car crashed into a police cruiser.
"Since all the other cameras were working, I'm sure that camera was working and it (the shooting) was edited out or that officer turned it off on purpose," said Ja'Mal Green, an activist who spoke at the rally.
"If this is brand new equipment, how come the other officers knew to turn their cameras on and the officer who shot the fatal shot failed to turn his on or it got mysteriously turned off?"
Release of the O'Neal shooting video was the first under a new policy that calls for such material to be made public within 60 days.
The policy was changed after public outrage last year following months of delay in releasing video that showed black teenager Laquan McDonald being shot 16 times by a white officer.
