(Reuters) - Amid public outrage over the fatal shooting of an Australian woman by Minneapolis police, the most persistent question was why officers did not turn on body cameras that could have captured what happened.

Don Damond is comforted by his son Zach Damond as he speaks to the media about his fiance, Justine Damond who was fatally shot by Minneapolis police in Minneapolis, Minnesota. REUTERS/Adam Bettcher

Experts on police procedure said the most common reasons for failure to turn on cameras nationally were officers’ forgetting or getting caught by surprise, not trying to hide something. The American Civil Liberties Union said the case showed a need for better compliance and training.

Justine Damond, who was originally from Sydney, was shot around midnight on Saturday by an officer responding to an emergency call she had placed about a possible assault behind her house in a quiet residential neighborhood.

Authorities said the officer shot the 40-year-old woman through the window as she approached his patrol car. Neither the officer’s body camera nor a dashboard camera were turned on, depriving authorities of potential evidence.

“There’s a knee jerk assumption that something nefarious is occurring” when cameras are not turned on, said spokesman Steve Tuttle of Axon Enterprise Inc, a leading maker of body cameras formerly called Taser International and the manufacturer of the equipment used in Minneapolis.

Malfunctions are rare, but when police are facing lethal danger, an officer is “not going to call time out” to turn on the camera, he said.

Officers currently must press a button to enable the cameras to record video and audio, and Axon will soon release a sensor that will trigger cameras to turn on when a gun is taken from its holster, he said.

The Minnesota Bureau of Criminal Apprehension, which reviews shootings involving the police in Minneapolis, is seeking any civilian video of the incident. It said the officer who shot Damond, who has been identified Mohamed Noor, and the officer in the patrol car with him, Matthew Harrity, have been placed on administrative leave.

Police have declined comment on questions about the cameras, pending an investigation. The lack of video footage has led the city’s mayor to call for a probe, while the American Civil Liberties Union suggested the police violated policy by failing to switch on the cameras.

Advocates said that when used, cameras protect both officers and the public and in some cases have reduced use of force and complaints against police.

“They should be on, every time,” said Steve Soboroff, a member of the Los Angeles Police Commission. Rigorous training and accountability is needed to ensure compliance, especially when police officers are under pressure.

USE CAMERAS ‘WHEN SAFE,’ GUIDELINES SAY

The technology has been adopted by police departments across the country since 2014 after a police shooting of a Missouri teenager sparked nationwide demonstrations over police treatment of minorities. Video evidence might have clarified whether the officer involved in that incident was justified in the shooting.

Minneapolis rolled out cameras late last year, and the department adopted guidelines calling for officers to activate them “when safe” in a variety of situations including traffic stops, emergency responses, vehicle pursuits, searches and before any use of force or contact with citizens.

There is only spotty national evidence available about how frequently police fail to turn on cameras. An Arizona State University study in 2014 found fewer than half of police incidents in Phoenix were recorded.

At least 14 people were killed by officers wearing body cams that were either not turned on or inoperative since 2014, the ACLU said in December. Even so, that was a tiny fraction of police-involved shootings.

Jim Pasco, a senior adviser with the Fraternal Order of Police, said non-compliance among police is not widespread, and a Pew Research Center report earlier this year found 66 percent of police supported the use of body cameras.

ACLU analyst Jay Stanley pointed to a KSTP-TV report based on Minneapolis police data showing officers on average uploaded no more than 6.1 hours of camera footage in March.

The department on Tuesday said the data was part of a study under way and that no conclusions had been reached.

Despite legitimate circumstances when police cannot turn on cameras, the Minneapolis shooting illustrates the need to boost training and compliance.

“Training is critical,” said Samuel Walker, a professor emeritus of criminal justice at the University of Omaha. “This is a relatively new experiment, and there is a learning curve.”