LOS ANGELES >> The critical moment when a gunman opened fire on two San Diego police officers, killing one, may never be seen. The surviving officer only activated his camera after the wounded shooter was running away.

San Diego is among departments with policies calling for officers to turn on cameras before initiating contact with a citizen in most cases. But like other departments, compliance is less than perfect.

The result is inconsistent use of an increasingly common tool meant to give investigators and an often-skeptical public a fuller picture of police actions.

“The main motive of body cameras is to provide openness and transparency, and build trust in the police,” said Samuel Walker, a retired criminal justice professor at the University of Nebraska-Omaha.

“If officers are not turning cameras on, well, you’re not going to build trust,” he said. “You’re going to reinforce the cynicism that already exists.”

He pointed to a study that showed across-the-board low compliance rates of officers in one high-crime Phoenix neighborhood between April 2013 and May 2014, the most recent information available. Officers only recorded 6.5 percent of traffic stops even though the department’s policy required cameras to be activated “as soon as it is safe and practical,” according to the study, conducted by Arizona State University’s Center for Violence Prevention and Community Safety.

The biggest part of the problem, Walker said, is a lack of discipline.

Chicago, Dallas, Denver, New Orleans, New York, Oakland and San Diego are among the cities that don’t specify penalties when officers fail to record, according to the Brennan Center for Justice at New York University’s School of Law.

The American Civil Liberties Union has studied the issue and said clear policies are vital, along with punishment for failure to comply.

“Departments can’t look the other way when officers fail to activate body cameras in critical incidents, or they become useless for accountability,” said Peter Bibring, a senior staff attorney at the ACLU of Southern California.

San Diego police have been criticized for failing to record a number of high-profile shootings. That prompted the department to revise its policy to stipulate that officers must turn on their cameras before most types of contact with citizens, but violations have continued.

Last week, the two San Diego gang unit officers on nighttime patrol pulled up next to a pedestrian on a darkened residential street, and the man almost immediately opened fire, police said. The suspect, Jesse Gomez, shot Wade Irwin as he got out of the patrol car and then fired through the open door and fatally wounded Irwin’s partner, Jonathan De Guzman, according to police.

Irwin fired back and started manually recording after the shooting, but police haven’t said what was captured.

The cameras are on before an officer hits record, and have a recall function to get video from shortly before an officer starts recording. That function allows 30 seconds to be retrieved, without audio.

It’s unclear if Irwin activated that feature.

Both Irwin and Gomez were seriously wounded and remain hospitalized.

Victor Torres, a leading civil rights attorney in San Diego, said the department’s policy makes it clear both officers should have been recording before approaching Gomez.

Police Chief Shelley Zimmerman has commended Irwin’s actions, including activating his body camera when he did, as heroic.

The Alameda County Sheriff’s Department changed its body-camera policy following a highly publicized incident last November where two deputies were caught on surveillance video using their batons to beat a car theft suspect in the middle of a street in San Francisco’s Mission District.

Eleven officers in all responded and 10 failed to turn on their body cameras. The one who did activate his did so by accident.

Three officers were placed on leave, including two who are charged with assault under color of authority.

No one was disciplined for failing to turn on their cameras because the department’s policy at the time encouraged, but did not require, their use, said Sgt. Ray Kelly, an agency spokesman. The agency now requires deputies to use the cameras in most circumstances and lays out the discipline for failure to comply.

The department hasn’t had a problem with compliance since, Kelly said.

Some departments are tapping new technology to take the human factor out of body cameras. Los Angeles will be among a handful of departments nationwide to deploy cameras made by Scottsdale, Arizona-based Taser International that begin automatically recording once signaled, such as when a patrol car’s siren is turned on or when a shotgun is taken out of its mount

“I believe by the end of three years these things will be built into a badge,” said Steve Soboroff, vice president of the civilian oversight board of the Los Angeles Police Department. “These cameras now, they’re like the old 10-pound cellphones.”

Kelly said his department also is looking at the new technology.

“The body camera is really new to law enforcement,” he said. “There are a lot of privacy concerns and body cameras don’t always accurately depict what an officer is seeing. But they are a great tool and they are the future. And they’re here to stay.”