The undocumented immigrant acquitted by a jury of murder in the shooting death of Kate Steinle on a San Francisco pier, in a case that touched off a furor about immigration and sanctuary laws, has appealed his conviction for illegally possessing the fatal weapon, saying momentary and accidental possession of a gun is not a crime.

The November 2017 verdict in the case of Jose Ines Garcia Zarate showed that jurors believed he “did not commit a willful act in firing the gun — that it went off accidentally just as the defense contended,” Garcia Zarate’s lawyer, Cliff Gardner, said in a filing Friday with the First District Court of Appeal in San Francisco.

Because Superior Court Judge Samuel Feng refused to instruct the jury that “momentary possession” of a gun is not a crime, the jury had no choice but to convict Garcia Zarate of illegal firearms possession by a previously convicted felon, Gardner said. He asked the court to overturn the conviction and order a new trial.

Garcia Zarate, 46, was sentenced to prison on the gun conviction and remains jailed to await deportation and possible federal prosecution.

Steinle, 32, was fatally wounded as she walked with her father on Pier 14 in July 2015. The bullet bounced off the pier’s concrete floor before striking her in the back. Garcia Zarate, who was holding the gun when it went off, had been released from federal prison four months earlier after serving 46 months for felony re-entry to the U.S. following a previous deportation to Mexico.

Immigration officials had transferred him to San Francisco County Jail because of a local warrant alleging he had fled to avoid marijuana charges in 1995, but city prosecutors dropped that case, and the Sheriff’s Department then released Garcia Zarate despite a federal request to hold him for deportation. Then-Sheriff Ross Mirkarimi said he interpreted the city’s sanctuary policy, which limited local cooperation with immigration enforcement, as barring him from notifying federal agents.

The handgun used in the Steinle killing had been stolen four days earlier from the car of a U.S. Bureau of Land Management ranger. It has not been established who stole the weapon. Prosecutors contended Garcia Zarate had brought the weapon to the site of the shooting, but his lawyers said it was wrapped in a T-shirt or cloth that he picked up from underneath a bench, unaware of its contents, and threw it in the bay after it unexpectedly went off.

After nearly six days of deliberations, the jury acquitted Garcia Zarate of murder, manslaughter and assault charges, and convicted him only of illegally possessing the gun. Garcia Zarate was sentenced to three years in prison, a term he has already served, but he remains in jail awaiting further proceedings.

President Trump called the verdict a “complete travesty of justice,” and his administration filed federal gun charges that could extend Garcia Zarate’s confinement by up to 10 years before he is deported. A federal judge in San Francisco has postponed his trial, however, to await a U.S. Supreme Court ruling on federal prosecutors’ authority to file charges that duplicate a state prosecution.

In appealing the gun conviction, Gardner, the defense lawyer, noted that jurors had asked Feng during deliberations whether knowingly possessing a gun was enough for a conviction and whether there was “any time requirement for possession.”

Gardner said the judge should have instructed the jury, as the defense requested, that Garcia Zarate was not guilty if he had possessed the gun “for a momentary or transitory period,” solely to dispose of it, and had not intended to prevent police from seizing it.

Feng rejected that instruction, citing evidence that Garcia Zarate had told a police officer he fired the gun. But Gardner said there was strong evidence that “he disposed of the firearm as soon as he learned what it was.”

Bob Egelko is a San Francisco Chronicle staff writer. Email: begelko@sfchronicle.com Twitter:@BobEgelko