The shot's origin is not in dispute, but Garcia Zarate's intentions are. According to instructions read to the jury, if Steinle's killing on July 1, 2015, was willful, deliberate and premeditated, it would justify a first-degree murder conviction. If her killing was the result of a dangerous act likely to cause someone's death, the jury could find Garcia Zarate guilty of second-degree murder. If he didn't intend to kill anyone but was criminally negligent in handling a loaded firearm, he could be convicted of involuntary manslaughter.
The jury of six men and six women also will consider the separate charges of being a felon in possession of a handgun and assault with a semi-automatic weapon.
The defense has said throughout the trial that Garcia Zarate -- impoverished and homeless on the streets of San Francisco for over two months -- picked up an unknown object that was wrapped in cloth while he was sitting on San Francisco's Pier 14. He didn't know what he was holding until it went off, the defense argues.
Prosecutor Diana Garcia said that interpretation of the evidence is not reasonable.
"This gun had value to him because it gave him power," she told the jury. "He wanted to fire this gun. ... Who's he going to shoot? It's a big game to him."
The gun was stolen four days before Steinle's death from a Bureau of Land Management ranger who had left the weapon in his car. There is no evidence tying Garcia Zarate to the auto burglary, and the jury was instructed to assume he didn't steal the gun.
But according to prosecutor Garica, he had the gun with him when he walked onto the pier. She showed photographs taken by a tourist that incidentally captured Garcia Zarate sitting in one of eight rotating metal chairs. She said one shows him "leering" at a woman as she jogged by. Another shows him turned, appearing to look in the direction of Steinle as she took a photo of her father and a family friend.
"Look where he's looking -- in her direction, right at her," Garcia said. "He's deciding where he wanted to fire that gun. This is only minutes before he fired it."
Garcia said the defendant "was trying to shoot someone without getting caught" and "jerked the trigger" as he tried to aim the gun while he held it between his knees. A since-retired San Francisco crime scene investigator testified earlier in the trial that "jerking the trigger" would cause the gun's muzzle to drop, firing the bullet lower than the shooter intended.
The bullet ricocheted 12 to 15 feet from Garcia Zarate and flew 78 more feet before it struck Steinle in the lower back.
Lead defense attorney Matt Gonzalez cautioned the jury in his closing argument against believing the prosecution's conclusions about the evidence.
"Be careful when you're asked to draw wild interpretations about his desire to hurt someone he doesn't know," Gonzalez told the jury. "You cannot take these trivial circumstances and decide that the conclusion they reach is one pointing to guilt."
Gonzalez said the case never should have been charged as murder, and he has said previously that it's the only case involving a ricochet shot charged as a murder in San Francisco history.
"He missed Ms. Steinle by 78 feet," Gonzalez said. "But for that ricochet, he did not hit her."
Most of the evidence presented during the defense's case went toward supporting some of Garcia Zarate's statement to homicide inspectors the night of Steinle's death. He was arrested within an hour of the shooting and repeatedly told police he found the gun at the pier. He said it was wrapped in cloth and that he threw it into the bay to stop it from continuing to fire.
"You know that that's ridiculous," prosecutor Garcia told the jury. "It was not going to keep going off by itself. It's not a firecracker."
She pointed out that he was sitting on the pier for 23 minutes before firing the fatal shot, throwing the gun into the bay and walking away.
Police divers recovered the gun the day after Steinle was killed, and it has been displayed to the jury several times.
The defense wants jurors to test-fire the gun and determine for themselves how easy or hard it would be to pull the trigger accidentally. Gonzalez told the jury they could demand the opportunity once they begin deliberations.
"That would give you a real sense of what this trigger pull is like," Gonzalez said.