SAN FRANCISCO — Guns don't go off without someone pulling the trigger, a retired police investigator testified Monday at the San Francisco murder trial of a Mexican national at the heart of a nationwide debate over immigration policy.

Jose Ines Garcia Zarate is charged with shooting to death Kate Steinle as she walked on a San Francisco pier with her father on July 1, 2015. Zarate had been deported five times was wanted for a sixth deportation before the shooting.

The San Francisco sheriff's department released him from jail despite a federal immigration request to detain him. San Francisco is a so-called sanctuary city that bars city officials from cooperating with federal immigration deportation efforts. President Donald Trump has threatened to withhold federal funding from cities with similar policies.

Garcia Zarate claims the shooting was accidental. He said a gun he found wrapped in a sheet on the pier accidentally fired when he picked it up. His lawyer, Matt Gonzalez, told jurors last week that Garcia Zarate didn't know he picked up a gun until it fired.

Retired San Francisco police inspector John Evans conceded during cross examination that he doesn't know whether Garcia Zarate fired the gun accidentally. But he did argue that accidental discharges result from a shooter mishandling a gun and pulling the trigger.

Evans said he prefers the term "negligent discharge" rather than "accidental discharge."

The semi-automatic handgun used to kill Steinle was stolen from a federal Bureau of Land Management ranger a week before the shooting. San Francisco Police Department officers carry similar weapons, and Gonzalez has argued that the gun is designed to fire with the slightest pressure. The department's officers reported 29 accidental weapon discharges 29 times between 2005 and 2011, he said.

Evans said guns "do not fire by themselves" and even accidental discharges require a trigger to be squeezed. Evans said he believed Garcia Zarate pointed the gun at Steinle and pulled the trigger.

Earlier, Evans testified that the bullet that killed Steinle ricocheted off the pier's concrete walkway. Gonzalez said the ricochet supports the accidental shooting argument.

But Evans said that inexperienced shooters often pull the trigger too hard, causing the barrel to dip before firing.

Gonzalez called that aspect of Evans' analysis "highly speculative." The two wrangled over whether the shot had traveled straight, which would support the prosecution's contention that Garcia Zarate aimed the gun before firing.

Prosecutors and Gonzalez said the case boils down to whether Garcia Zarate pointed and fired the gun intentionally or the weapon accidentally discharged.

The shooting sparked a political furor during last year's presidential race, with then-candidate Trump citing the killing as a reason to toughen U.S. immigration policies.

Garcia Zarate had been released from the San Francisco jail about three months before the shooting, despite a request by federal immigration authorities to detain him for further deportation proceedings.

Garcia Zarate was arrested shortly after Steinle died in the arms of her father, who has attended nearly every day of the trial with his wife and son.

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This story has been corrected to say the bullet traveled 100 feet, not yards