Los Angeles police officers tampered with voice recording equipment in dozens of patrol cars in an effort to avoid being monitored while on duty, according to records and interviews.

An inspection by Los Angeles Police Department investigators found about half of the estimated 80 cars in one South L.A. patrol division were missing antennas, which help capture what officers say in the field. The antennas in at least 10 more cars in nearby divisions had also been removed.

LAPD Chief Charlie Beck and other top officials learned of the problem last summer but chose not to investigate which officers were responsible. Rather, the officials issued warnings against continued meddling and put checks in place to account for antennas at the start and end of each patrol shift.

Members of the Police Commission, which oversees the department, were not briefed about the problem until months later. In interviews with The Times, some commissioners said they were alarmed by the officers’ attempts to conceal what occurred in the field, as well as the failure of department officials to come forward when the problem first came to light.

“On an issue like this, we need to be brought in right away,” commission President Steve Soboroff said. “This equipment is for the protection of the public and of the officers. To have people who don’t like the rules to take it upon themselves to do something like this is very troubling.”

Beck said there was no deliberate attempt to keep the commission in the dark, saying the failure to alert the board was “unintentional.”

“The department did not try to hide this issue,” Beck said, emphasizing that he has been a vocal advocate for the in-car video cameras that rely on the antennas.