After Minneapolis police officers fatally shot an Australian woman who had called 911 over the weekend, her friends, relatives and neighbors wanted to know what happened. Her fiance, in an emotional news briefing this week, pleaded for any answers about how Justine Damond wound up dead.

But even though officials on Tuesday gave a brief account of the shooting – saying that officers were startled by a loud noise right before Damond approached their car – her final moments remain shrouded in mystery, in part because the two officers who encountered Damond did not turn on their body cameras as they arrived at the scene, preventing any video from capturing what happened.

The shooting in Minneapolis on Saturday has renewed scrutiny of how police in the Twin Cities use force. The incident also highlighted the limitations of police-worn body cameras, which reform advocates and law enforcement officials alike have championed as a way to help repair trust between officers and communities, something Minneapolis officials cited when rolling them out last year.

“Officer-worn body cameras are merely a tool for improving police-community relations; they are not a solution in themselves,” Janeé Harteau, the Minneapolis police chief, and Betsy Hodges, the mayor, wrote in a July 2016 letter announcing that all officers would begin wearing them. “But body cameras are an important tool, one that will help us continue to transform the relationship between police and community for the better.”

Yet even in departments that have adopted body-worn cameras – like Minneapolis, where all patrol officers now wear them, along with SWAT members – the devices have inherent limitations, which might have contributed to the lack of footage capturing the moments surrounding Damond’s death. Key among them: In many departments, including in Minneapolis, body cameras must be manually turned on, which means they rely on officers to activate them.

In Damond’s case, the body cameras were not turned on until after the shooting, according to investigators; so far, the only details about what happened have been provided by one of the officers in the car.

Specific policies on body cameras vary from department to department, but in Minneapolis, the police department’s policies for body-worn cameras lay out a list of reasons for officers to start recording. The policy states that “when safe to do so,” officers must turn on the body cameras during traffic stops, contact that is adversarial and searches of vehicles and buildings. They also are required to activate the body cameras before using force, and if not beforehand, “as soon as it is safe to do so.”

It is still unclear why the two officers did not activate their cameras on Saturday night as they responded to a call about an assault. State investigators have said only that the cameras were not on at the time of the shooting, but they have not said why. (The squad car camera also was not turned on, investigators said late Tuesday.) The Minneapolis Police Department has declined to answer questions about whether the officers violated departmental policy and whether that is under investigation, citing a state probe into the shooting.

Even though the Minneapolis body cameras have to be turned on, there is a failsafe for when officers only start filming after something takes place. The cameras worn by Minneapolis officers come with what is known as a “buffer time” function, which means they are always filming but almost never actually saving the footage. When the cameras are manually activated, they save the 30-second clip filmed right before the officer started recording. (Similar technology is used within some public transit agencies, like several in the Washington region, which use constantly filming cameras that perpetually record over old footage unless the system is triggered and begins saving those clips.)

Body cameras worn by officers in Minneapolis are made by Axon (also known as Taser International, which has changed its name, along with its focus, as it pivots from handheld electrical shock devices to the lucrative body camera market). One day before the letter from Harteau and Hodges declaring that body cameras were coming to Minneapolis last year, Axon announced that it had sold more than 600 Axon Body 2 cameras to the city’s police department.

Though the Axon Body 2 allows that “buffer time” – the period when it is always filming, but only saving when activated – to be extended as long as two minutes, according to Axon’s website, a Minneapolis police spokesman said the cameras worn by the department’s officers have a 30-second buffer time.

Damond’s death is not the first controversial police shooting involving an officer who wore a body camera but failed to capture the pivotal moments. In perhaps the most high-profile case, one of the Charlotte police officers who fatally shot Keith Scott there last year, setting off intense demonstrations, failed to activate his body camera as soon as he responded to the situation, violating departmental policy. Because the 30-second “buffer” footage recorded before the shooting has no audio, investigators and the public could not see key details about what preceded that shooting. (The officer was cleared by a prosecutor last November.)

More fatal shootings by police have been captured on video recently, including both bystander video and footage captured by officers’ body cameras. Police departments have been equipping officers with body-worn cameras for nearly a decade, but this push expanded after protests erupted in Ferguson, Mo., in August 2014, the beginning of a wave of demonstrations nationwide. Michael D. White, an Arizona State University professor who has researched police implementation of body cameras, told The Washington Post last year that up to half of the 18,000 police departments nationwide have officers who wear body cameras.

Even when videos exist, police departments have pushed against releasing video footage too early in their investigations. And just because an officer was wearing a body camera does not mean it recorded key incidents. Case in point: In at least a dozen fatal shootings last year, officers were wearing cameras that failed to record fatal encounters, according to a Washington Post survey of departments. Some of these included among the most well-known police shootings in recent years, such as Alton Sterling’s death in Baton Rouge, where both officers’ body cameras fell off during the encounter.

Experts have cautioned that video footage, even when it captures critical moments of deadly encounters, can still be incomplete. Having body camera footage alone might not say everything about what happened before the recording or appropriately capture an officer’s frame of mind. In Ohio, explicit body-camera video showed an officer fatally shooting a driver during a traffic stop in 2015, drawing curt condemnation from a prosecutor; on Tuesday, after two mistrials, the same prosecutor said he was giving up on the case because some jurors convinced him they would never vote to convict.

“Video is not a magic solution,” David A. Harris, a law professor at the University of Pittsburgh and an expert in police use of force, told The Post last month. “Sometimes it will be very helpful. And sometimes it will put certain facts, even if not all the facts, but certain facts beyond dispute. But many times, it does not put enough of the facts beyond dispute.”

According to The Washington Post’s database tracking police shootings, there have been at least 547 fatal shootings by police this year, and 57 of them involved police-worn body cameras, on pace to trail last year but to be up from two years ago.

There were 963 fatal shootings by police last year, and 138 of them involved officers wearing body cameras. In 2015, the first year The Post began tracking such shootings, there were 991 fatal shootings by police with 74 of them captured on video.

Policies on body cameras can vary depending on department, and in a 2014 report, the Police Executive Research Forum (PERF) found numerous agencies that did not have written policies in place governing body cameras. Few of the departments surveyed by PERF recorded all encounters with the public, and it was more common for departments to activate them when responding to calls for service or during law-enforcement related encounters.

PERF recommended that officers “be required to activate their body-worn cameras when responding to all calls for service and during all law enforcement-related encounters and activities that occur while the officer is on duty.”

The American Civil Liberties Union said in 2015 that the ideal policy for body-worn cameras would be “continuous recording throughout a police officer’s shift,” though the group acknowledged that this raised privacy issues for the public and officers alike. So instead, the ACLU recommended recording all public encounters.

In Minnesota, the officers’ failure to record the encounter has drawn criticism. The American Civil Liberties Union of Minnesota said the officers “thwarted the public’s right to know what happened to Ms. Damond and why the police killed her.”

Hennepin County Attorney Mike Freeman, whose office will decide whether the officers involved will face charges, similarly said he felt the cameras should have been filming.

“I think they should be turned on,” Freeman said Monday, according to the Minneapolis Star Tribune. “I do understand this, they were driving down an alley, the victim approached the car. That’s not necessarily a time you must, but frankly I think it’s a time you should.”