Four San Francisco police officers will not face charges for shooting and killing Alejandro “Alex” Nieto last year in Bernal Heights Park, because Nieto pointed a Taser shock weapon that the officers reasonably mistook for a pistol, the district attorney’s office said Friday.

The officers fired a total of 59 shots, District Attorney George Gascón said in a letter to Police Chief Greg Suhr. Two later-arriving officers opened fire on Nieto after they heard the popping of their colleagues’ gun blasts and believed Nieto was firing back, the letter said.

But Gascón’s report said all four officers had “continued to believe their lives were in danger ... until Mr. Nieto’s head and weapon went down.”

The four — Lt. Jason Sawyer and Officers Roger Morse, Richard Schiff and Nathan Chew — had responded to witness reports that Nieto, a 28-year-old Mission resident, had a gun and was acting erratically on March 21.

The officers “saw what appeared to be a gun as soon as they encountered him on a hill,” the district attorney’s report said. “Mr. Nieto refused to obey orders to show his hands and, instead, immediately drew and pointed his weapon at the officers.”

At a news conference, Gascón added, “Given the circumstances, I’m not sure this was a preventable shooting.” In such instances, he said, “a shooting is likely to occur. It is lawful. It is clearly constitutional, and I’m not sure there is much that could have been done to prevent it.”

Nieto’s friends and others have demanded punishment for the officers in the shooting, calling it unnecessary and excessive. They say Nieto had been carrying a Taser before his shift as a nightclub security guard, and would never have pointed the weapon at officers.

Benjamin Bac Sierra, a friend of Nieto’s, said Friday that the findings were “not unexpected” but included “grave inconsistencies” compared with what Suhr has told the public.

'Broken system’

Adriana Camarena, a supporter of Nieto’s family, said, “This is a broken system. It’s delusional to think we’ll get a different result when you have a former chief of police investigating a police shooting.”

Suhr succeeded Gascón as chief of police in 2011.

Nieto, a City College student, was struck by 10 to 15 gunshots, according to his autopsy report. In the report, the city medical examiner highlighted Nieto’s history of aggressive and bizarre behavior, auditory hallucinations and failure to take prescribed psychiatric medications. In 2011, Nieto was twice placed on a psychiatric hold, once after trying to burn down his parents’ home, authorities said.

Accused of firing Taser

Gascón’s report to Suhr provided the most detailed account by authorities of the shooting. It said that on March 3, less than three weeks before he was killed, Nieto was accused of firing a Taser at the estranged husband of a friend.

On the day of the police shooting, prosecutors said, several dog walkers at Bernal Heights Park reported feeling threatened by Nieto. One witness told investigators Nieto had pulled what looked like a pistol from his holster and pointed it at his dog — the animal had taken interest in a bag of chips Nieto was eating — before realizing it was a Taser.

'Aimed it at my dog’

“When he turned it into profile and aimed it at my dog, that’s when I could tell that it was a Taser just because of the size of the muzzle area,” the witness said.

The witness said he begged Nieto not to shoot his dog with the Taser, telling him the dog wasn’t aggressive and “just wants some chips,” according to the report. Nieto aimed the Taser at the witness and yelled expletives, the report said, before the witness left the park, while warning others about Nieto.

After Schiff and Sawyer arrived, in uniform, and ordered Nieto to show his hands, Nieto allegedly walked toward them, lifted his sweatshirt to expose a black holster, and shouted back at the officers, “Let me see your hands!” Nieto then “squared off with them in a defensive stance,” the report said, drew what turned out to be the Taser and pointed it directly at officers.

Schiff and Sawyer, who was then a sergeant, opened fire, authorities said. Nieto fell to the ground with his hands in front of him, police said. Schiff reported seeing a red light coming from the object Nieto was carrying and “thought it might be a laser sight. Both officers believed Mr. Nieto was still trying to fire back at them, and continued to fire,” the report said.

2 more officers fire

Morse and Chew soon arrived and joined in firing at Nieto, with Morse reporting that he “thought Mr. Nieto was firing at them when he heard popping sounds and saw what he thought were muzzle flashes,” the report said. “All four officers continued firing until Mr. Nieto’s head and weapon went down, and Sergeant Sawyer yelled for the officers to cease fire.”

Why 'so many bullets’

Adanté Pointer, the family’s attorney, said Friday that a civilian had recognized Nieto was carrying a Taser. Trained police officers who carry both guns and the shock weapons, he said, should know the difference as well.

“For the world of me,” Pointer said, “I cannot rationalize why the officers would fire so many bullets at a guy who clearly never fired back one shot.”

Henry K. Lee and Kale Williams are San Francisco Chronicle staff writers. E-mail: hlee@sfchronicle.com, kwilliams@sfchronicle.com Twitter: @henryklee, @sfkale