Experts say the absence of audio on police dash-cam video of a white Chicago officer fatally shooting a black teenager raises red flags.

Protocol requires that all cruisers record audio. But videos from four other squad cars the night officer Jason Van Dyke short Laquan McDonald were also silent.

That's raised questions about a possible cover-up.

Experts say it's plausible for a single car to have an audio-system glitch. But they couldn't imagine how a fleet of cars would all lose audio at the same time by mere happenstance.

Gregg Stutchman's specialized in video forensics for 23 years. He says he's never heard of audio failing on several police cars at the same incident.

He says the most likely explanation is that officers intentionally switched the audio off.