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CHICAGO (AP) — A police dash-cam video that captures a white Chicago officer fatally shooting a black teenager 16 times has no sound, nor do videos from four other squad cars at the scene. But department protocol indicates all the cruisers should have been recording audio that night.

The silence is almost sure to figure into the ongoing federal investigation of the case, and it raises questions about whether officers were careless with the recording equipment or, worse, attempting a cover-up.

“When you’ve got a standup cop with nothing to hide, the dash-cam is his friend,” said Gregg Stutchman, who has specialized in video forensics in California for 23 years. “But for cops who aren’t quite as standup, it would make sense that they wouldn’t what things recorded.”

Several experts on the type of equipment commonly installed in police vehicles told The Associated Press that it’s plausible for a single squad car to have a glitch preventing sound recording, but they could not imagine how an entire fleet of cars would ever lose audio at the same time and place by mere happenstance.

“I’ve never heard it before,” Stutchman said. “It raises a red flag.” The more likely explanation is that audio was intentionally switched off, he said.

The silent video at the core of the shooting case in Chicago shows officer Jason Van Dyke stepping out of his car on Oct. 20, 2014, and almost immediately opening fire on Laquan McDonald as he walks away from officers. Van Dyke continues to fire on the 17-year-old even after he crumples to the ground.

On the same day that the video was released last week by the city, the Cook County State’s Attorney’s Office charged Van Dyke with first-degree murder.

Dash-cam video and audio should switch on automatically when a vehicle’s emergency lights are activated. If the lights aren’t used during an incident, officers should turn on the system manually, according to a directive on the police department’s website.

Sound can be central to understanding what happened at a crime scene, said Michigan-based Ed Primeau, a Michigan-based audio and video forensics expert.

In the Chicago shooting, audio would have enabled investigators to gauge the level of anxiety among officers as they pursued McDonald, Primuea said. And sound could have been critical to piecing together how things played out in the minutes before the shooting as officers spoke to each other or to a dispatcher over their radios.

“That gives you a whole play-by-play, like who is coming around the corner in a chase or which officer is going where,” he said. “It can help with the entire narrative.”

Without sound, it’s hard to know just when the shooting started and stopped. Did the shots come in quick succession or was there a pause between bursts of gunfire? And just what did Van Dyke and fellow officers say in the seconds before Van Dyke opened fire? What, if anything, did McDonald say to them?

Neither police nor state prosecutors have offered any detailed explanation of why there is no audio with the video.

Police spokesman Anthony Guglielmi suggested that the absence of audio was probably an unforeseen technological fault. In an email to The Associated Press, he wrote: “As with any technology, at times software issues or operator error may keep the cameras from operating as they normally should.”

Ensuring the audio system is fully functional also requires that officers, usually at the start of their shifts, place a unit that includes a microphone and transmitter on their bodies and make sure it’s synced with the audio system, Stutchman explained.

Videos from five police cars have been released by the city, and none has functioning sound. Videos from three other cars that responded have not been released.

Patrol officers have never liked the idea of being electronically monitored, regarding it as something imposed from the upper brass, Stutchman said.

But, he added, in his experience, better officers are meticulous about making sure at the start of each shift that their audio and video equipment is working.

Darrel Stephens, executive director of the Major Cities Chiefs Association and a former police chief in North Carolina, echoed that. He said there are malfunctions, or in some cases, officers who forget to switch audio on.

“It’s not a perfect technology. There are times that it doesn’t work every well,” he said.

Texas-based Coban Technologies supplies the dash-cams to Chicago, as it does to Los Angeles. Messages seeking comment from the company were not returned Wednesday.

Some of the Chicago videos released do contain faint buzzes, some of which sound similar to an engine or a siren. Stutchman said those are probably electrical pulses picked up by the dash-cam system that are not actual audio but could mimic sounds of sirens or engines.

The U.S. Attorney’s Office has not specified what issues it’s investigating.

Primeau said it’s likely that federal authorities would bring in forensics experts to determine why no useable audio seems to exist, with other investigators probably reviewing logs for any notes about how the systems were operating before and after the shooting.

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Associated Press Writer Sophia Tareen also contributed to this report.

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Follow Michael Tarm on Twitter at http://twitter.com/mtarm .

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