SANTA ANA, Calif. -- An Anaheim police officer was justified and acted lawfully when he shot and killed a 25-year-old man in July in the first of two shootings that caused weeks of heated protests in the city's streets, the Orange County district attorney said after an investigation.

District Attorney Tony Rackauckas announced the findings in the July 21, 2012 shooting death of Manuel Diaz in a letter Wednesday.

Diaz's shooting by Officer Nicholas Bennallack and the next day's killing of another man by a different Anaheim police officer sparked a series of demonstrations, some of which turned violent.

Police said Diaz and his two companions ran when they were approached by officers. Police also said Diaz was a documented gang member from Santa Ana, an allegation his family has denied.

The letter of findings includes statements from the officers and witnesses, a rundown of the evidence, details from the autopsy and a summary of YouTube video taken after the shooting. It also includes details on Diaz's criminal history and Bennallack's job history, along with a legal analysis of the shooting.

"It is our legal opinion that the evidence does not support a finding of criminal culpability on the part of Officer Bennallack, and that there is significant evidence that the officer's actions were reasonable and justified under the circumstances when he shot Diaz."

The letter says Bennallack cannot be charged with a crime because the prosecution would be unable to prove beyond a reasonable doubt that the officer didn't act in self-defense or the defense of others when he shot Diaz.

"Diaz may have been holding a gun," the letter reads.

Three facts are provided to support this: Diaz had a prior felony conviction for possessing a gun for the "benefit of the gang," a cellphone found nearby included photos of Diaz posing with a gun days before, and a raid weeks later on gang members found 40 guns.

The findings also say Bennallack had good reason to believe Diaz posed a danger because witness statements indicate Diaz brought an object out of his waistband and raised it as he turned toward officers.

Though the object does not match the description of a handgun, the letter says there must be an "allowance for the 'split-second judgments in tense circumstances' required of Officer Bennallack."

Because Bennallack believed Diaz was pulling a gun from his waistband, the letter states it was "not disproportional" to respond with deadly force even if Bennallack could have responded with his stun gun instead.

Diaz's family has filed a $50 million wrongful death lawsuit against the officer and the city.