The man fatally shot by Los Angeles police officers "repeatedly refused to comply with officers demands," and evidence suggests he grabbed at a police officer's pistol before he was shot, Los Angeles Police Chief Charlie Beck told reporters.
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Beck showed photos of an officer's gun with alterations he said were consistent with a struggle over the weapon.
"As you can see, the slide is partially engaged and a round has been partially ejected and fouled the firing chamber," Beck said. "This is indicative of a struggle over the weapon. As you can also see, the magazine is dislodged from its resting place. This is also indicative of force used on the weapon. There's also a screen grab from a privately taken video. If you examine that, it appears that the suspect's hand is reaching for the officer's waistband in the area where his pistol would be located."
The officers involved in the shooting were not identified, but Beck said two were treated for injuries at a hospital and released, and one is now using crutches.
He added that officers first used a stun gun against the man, and then shot him when he continued to resist, after responding to a robbery call.
"It appears to me the officers acted compassionately up until the time when force was required," Beck said.
The investigation will take video footage of the event into account, including video from two body cameras that were worn by the officers involved in the incident, he added.
At least two of the officers had been through the most extensive mental illness training offered by the department and had experience working with the largely homeless population of the area, known as Skid Row, according to Beck.
"The Skid Row population is very transient, obviously, and this individual had only been there a short amount of time ... so I don't know about his familiarity to the officers," Beck said.
"This is an extreme tragedy," Beck added. "We feel great compassion in the LAPD for people who live in conditions of homelessness and often mental illness with no treatment."
The investigation into Sunday's fatal shooting may take up to five months, a LAPD source familiar with the incident told ABC News.
“These are some of the most exhaustive investigations we do,” Beck told reporters.
Whether or not the man tried grabbing one of the officers’ weapons is going to be at the heart of the investigation.
"When you see an individual moving that way with their arms flailing around, that’s a concern," said former LAPD officer Steve Gomez, who now works as a consultant for ABC News.
"If he happened to get his hand on one of the officers’ guns, that would trigger that kind of reaction from one of the officers," Gomez said.
The LAPD's specialized Force Investigative Division has already been dispatched to the scene of the Skid Row shooting to collect evidence and interview witnesses, the LAPD wrote in a statement on its blog, but another key portion of the investigation will be based on photos and videos of the altercation.
The first video posted to Facebook hours after the shooting has already received more than 5.2 million views, but that is not the only footage that will be carefully examined.
The source told ABC News that at least one of the officers involved in the altercation was wearing a body camera.
The LAPD's Office of the Inspector General will be coordinating with the FID team, which will then present its report to the LAPD Use of Force Board.
A separate investigation will also be conducted by the Los Angeles County District Attorney’s Justice System Integrity Division.
ABC News' Josh Margolin contributed to this report.