US President Donald Trump has condemned the acquittal of a Mexican man over the killing of Kathryn Steinle, whose death on a San Francisco pier touched off a national immigration debate two years ago.

Jose Ines Garcia Zarate was found not guilty of possible charges ranging from involuntary manslaughter to first-degree murder over the fatal shooting two years ago.

Garcia Zarate had been deported five times and was wanted for a sixth deportation when Steinle was fatally shot in the back while walking with her father on the pier.

The shooting came during the presidential primary campaign in July 2015 and touched off a fierce debate over the country's immigration policies in the presidential race the following year.

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"A disgraceful verdict in the Kate Steinle case! No wonder the people of our Country are so angry with Illegal Immigration," the US president tweeted.

The case spotlighted San Francisco's "sanctuary city" policy, which limits local officials from cooperating with US immigration authorities.

"From Day 1 this case was used as a means to foment hate, to foment division and to foment a programme of mass deportation. It was used to catapult a presidency along that philosophy of hate of others," defence attorney Francisco Ugarte said after the verdict. "I believe today is a day of vindication for the rest of immigrants."

Flowers and a portrait of Kate Steinle remain at a memorial site on Pier 14 in San Francisco. Photo / AP
Flowers and a portrait of Kate Steinle remain at a memorial site on Pier 14 in San Francisco. Photo / AP

US Attorney General Jeff Sessions said he hoped local officials would "consider carefully the harm they are doing to their citizens" by not cooperating with federal immigration officials.

Sessions said San Francisco's "decision to protect criminal aliens led to the preventable and heartbreaking death" of Steinle.

Politics, however, did not come up in the month-long trial that featured extensive testimony from ballistics experts. Defence attorneys argued that Garcia Zarate was a hapless homeless man who killed Steinle in a freak accident. Prosecutors said he meant to shoot and kill her.

Garcia Zarate did not deny shooting Steinle and said it was an accident. Jurors did find him guilty of being a felon in possession of a firearm, which Public Defender Jeff Adachi said carries a potential sentence of 16 months to three years.

"The verdict that came in today was not the one we were hoping for," said Alex Bastian, a spokesman for the San Francisco prosecutor's office. "The jury came back with the verdict they did, and we will respect that decision. ... This is really about the Steinle family. They showed incredible resolve during this whole process."

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Liz Sullivan and Jim Steinle, the parents of Kate Steinle who was killed by an undocumented immigrant, speak during a news conference in 2015. Photo / Getty
Liz Sullivan and Jim Steinle, the parents of Kate Steinle who was killed by an undocumented immigrant, speak during a news conference in 2015. Photo / Getty

The family did not attend the reading of the verdict. Jurors left without comment and the judge sealed their names.

San Francisco Deputy District Attorney Diana Garcia said during the trial that she didn't know why Garcia Zarate fired the weapon, but he created a risk of death by bringing the firearm to the pier and twirling around on a chair for at least 20 minutes before he fired.

"He did kill someone. He took the life of a young, vibrant, beautiful, cherished woman by the name of Kate Steinle," she said.

Defence attorney Matt Gonzalez said in his closing argument that he knew it was difficult to believe Garcia Zarate found an object that turned out to be a weapon, which fired when he picked it up.

But he told jurors that Garcia Zarate had no motivation to kill Steinle and that as awful as her death was, "nothing you do is going to fix that".

The bullet ricocheted on the pier's concrete walkway and fatally struck Steinle in the back.

The gun was stolen from the SUV of a US Bureau of Land Management ranger that was parked in San Francisco. The city has been plagued by an epidemic of car burglaries in recent years.

Before the shooting, Garcia Zarate had finished a federal prison sentence for illegal re-entry into the United States and had been transferred to San Francisco's jail in March 2015 to face a 20-year-old charge for selling marijuana.

The sheriff's department released him a few days later after prosecutors dropped the marijuana charge, despite a request from federal immigration officials to detain him for deportation.

Trump said during the presidential campaign that Steinle's death was another reason the United States needed to build a wall on its southern border and tighten its immigration policies.

Trump signed an executive order to withhold funding from sanctuary cities, but a federal judge recently blocked it in a lawsuit from two California counties, San Francisco and Santa Clara. The administration has appealed.