Two years after Alejandro “Alex” Nieto died in a hail of police bullets in a San Francisco park, a jury will hear strikingly different accounts this week of a shooting that ignited a series of protests — demonstrations that continued Tuesday morning outside the city’s federal courthouse as the civil trial opened.

The trial in his parents’ wrongful-death suit against the city of San Francisco, which accuses four police officers of using unreasonable force, is set to begin Tuesday with jury selection and opening statements.

It comes amid a national debate over police shootings, and it revolves around what Nieto, a 28-year-old community organizer and City College of San Francisco student with a history of erratic behavior, was doing with the Taser stun gun he carried for his job as a security guard when police pulled up and swiftly opened fire.

According to the officers, Nieto pointed the Taser at them, prompting them to mistake it for a gun and fire in self-defense. The district attorney’s office declined to file charges, saying the officers reasonably mistook the Taser for a pistol.

But while city attorneys say evidence backs up that assertion — the Taser’s electric darts had been fired, they say, and were found protruding from the weapon — Nieto’s relatives and friends believe he posed no real threat and that officers are covering up an unjustified barrage of 59 bullets.

“The Nietos deserve their day in court,” said Adante Pointer, the family’s attorney. “It will be up to the jury to decide if it makes any sense that officers would fire 59 shots at a person who didn’t fire one.”

Behavior in park

Nieto was killed on March 21, 2014, in Bernal Heights Park on Bernal Hill, where he sat watching the sunset and eating a burrito before his shift as a security guard began. According to court documents, police received reports of a Latino man in a red jacket with a gun acting erratically and, at one point, pointing his weapon at a dog trying to eat his chips.

At about 7:10 p.m., Lt. Jason Sawyer and Officer Richard Schiff responded to the reports, pulling up the park path in their patrol car. They said they came across Nieto — and spotted a bulge on his right hip.

The officers stopped their car in his path and exited with guns drawn, instructing Nieto to show his hands. According to the officers, Nieto was about 100 feet away when he countered, “No, show me your hands,” before pulling out his weapon and pointing it at them.

On the ground

The Taser had a laser sight, similar to some firearms, the city said. Sawyer and Schiff said they believed Nieto was about to start shooting at them — and had to open fire to save themselves. Nieto went to the ground, but the officers said he ended up on his stomach, with his arms out and weapon up, prompting them to fire more rounds.

Two later-arriving officers, Roger Morse and Nathan Chew, shot at the fallen man as well. They said they heard the popping of their colleagues’ gun blasts, saw the Taser’s laser sight pointed at them, and saw what they believed to be muzzle fire coming from Nieto. The officers were mistaken, but the district attorney’s office described this second volley of shots as reasonable given the circumstances.

The officers — one of whom shouted “cease fire” — then reported approaching Nieto and kicking the weapon, which they now saw was a Taser, out of his hand. Nieto had been shot at least 14 times.

The district attorney’s office noted that the Taser’s electrodes had been deployed, indicating someone had pulled the trigger. A memory chip in the Taser showed it was on, and that the trigger was pulled three times at roughly the time of the police shooting, according to a Taser International analyst who is scheduled to testify at the trial.

To city attorneys, this is proof that Nieto drew the Taser in a menacing manner. But to his family, the facts don’t add up.

The Taser was off when it was taken into evidence. Pointer, the family attorney, said the pinpointing of what time the stun gun’s trigger was pulled is unreliable — an analyst had to correct it because the clock was several minutes slow, and had to calibrate it from Greenwich mean time.

Indications of firing

Moreover, Pointer said, if the Taser had been fired at the scene, police would have found “blast doors and confetti pieces of paper that come out the end of the Taser.”

The city said the Taser could have gotten turned off when an officer kicked it out of Nieto’s hands, and the district attorney cited high winds the night Nieto died, which could have blown away the confetti.

Another central dispute in the case will be aired when Pointer calls a witness, Antonio Theodore, who said he was on Bernal Hill and saw Nieto with his hands in his jacket pockets when the officers started firing.

In court documents, though, city attorneys questioned Theodore’s credibility, saying he was two football fields away, was not wearing his glasses, and asserted that the officers shot Nieto with rifles, when they actually used pistols.

One thing the jury will not hear about is Nieto’s psychiatric history, which the judge ruled inadmissible — because the officers did not know his history at the time of the shooting.

In clearing the four officers, who have since returned to duty, city prosecutors cited Nieto’s past bizarre behavior, hallucinations and failure to take prescribed psychiatric medications. In 2011, he was twice hospitalized involuntarily, once after trying to burn down his parents’ home. Three weeks before the shooting, Nieto was accused of firing a Taser at the estranged husband of a friend.

‘Racial bias’

But to Adriana Camarena, a supporter of Nieto’s family, the shooting by police was not a result of mental illness but racial profiling gone awry — starting with the park-goers feeling uneasy around a Latino man and reporting him to police, and ending tragically with the officers who saw Nieto in a red jacket and assumed he was an armed gang member.

“We believe he never posed a threat because he was an upstanding community member,” Camarena said. “The people who called 911 never had an interaction with Alex. They were never threatened by Alex. What horrifies us is that the police, rather than do their jobs and investigate the call, they responded with that same racial bias. Since then, they’ve been covering up an unlawful killing.”

Margaret Baumgartner, the deputy city attorney defending the officers, described them as “remarkably dedicated.” Sawyer, who was promoted to lieutenant after the Nieto shooting, is a three-time recipient of a medal of valor, once for his actions in fatally shooting a man who had allegedly used his Mercedes-Benz to pin another office to a parking meter.

Troubling timing

But she said the timing of the trial is difficult, coming in the wake of the controversial San Francisco police shooting of Mario Woods in the Bayview neighborhood, which spurred a U.S. Department of Justice review of Police Department practices. On Friday, hundreds of Nieto’s supporters marched for another young Latino man fatally shot by police in the Mission District, Amilcar Perez-Lopez.

“The hardest thing about this case is overcoming people’s presumptions that police officers do bad things,” Baumgartner said. “I have very little concern about the facts of the case. What I have concern about is people’s presumptions about police officers.”

Vivian Ho is a San Francisco Chronicle staff writer. Email: vho@sfchronicle.com Twitter: @VivianHo

Scene of the Nieto killing

A rendering of the scene after the March 21, 2014, fatal officer-involved shooting of Alejandro Nieto, based on information taken from court documents.