Jose Ines Garcia Zarate, 54, admitted to killing Steinle as she walked the San Francisco waterfront with her father in the summer of 2015 but called the shooting a shocking accident.

The case took on outsized significance, though, when it was revealed thatGarcia Zarate was an illegal immigrant and repeat felon who had been deported five times and sneaked back into the U.S. each time. He was protected from another deportation by San Francisco’s sanctuary policy restricting cooperation with federal immigration authorities.

Donald Trump, new to the Republican presidential race at the time, quickly seized on the case and used it to build a winning campaign, complaining of rapists and other bad elements coming from Mexico and vowing to build a border wall to stop them.

On Thursday, Garcia Zarate’s attorneys said the verdict was boost for immigrants and a slap at Mr. Trump.

“From Day One, this case was used as a means to foment hate, to foment division, to foment a program of mass deportation. It was used to catapult a presidency along that philosophy of hate for others,” said Francisco Ugarte. “Today is a vindication for the rights of immigrants.”

Steinle’s father, Jim, who watched his daughter collapse in his arms after she was shot, said the decision was the latest failure in a long series that led to her death.

“We’re just shocked — saddened and shocked. … That’s about it,” the father told the San Francisco Chronicle after the verdict. “There’s no other way you can coin it. Justice was rendered, but it was not served.”

Garcia Zarate was found guilty of being a felon in possession of a firearm, but he was acquitted of murder and manslaughter charges in a case that delved deeply into the weapon he said he found, and whether he intended to aim it at the attractive 32-year-old woman walking with her father.

Prosecutors argued that Garcia Zarate should have been convicted because he created the conditions for Steinle’s death by having the gun and handling it cavalierly.

The Sig Sauer pistol had been stolen from a federal Bureau of Land Management ranger’s vehicle. Garcia Zarate, who went by the name Juan Francisco Lopez-Sanchez at the time, said he found it on the waterfront pier.

Experts on both sides debated whether the gun was prone to accidental fire or required intention.

While neither the prosecution nor defense delved into politics during the trial, commentators said the critical issue was Garcia Zarate’s status as an illegal immigrant. If he had been deported and kept out of the U.S., they said, then the shooting would never have happened.

Attorney General Jeff Sessions laid blame at the feet of San Francisco.

“San Francisco’s decision to protect criminal aliens led to the preventable and heartbreaking death of Kate Steinle,” Mr. Sessions said.

“I urge the leaders of the nation’s communities to reflect on the outcome of this case and consider carefully the harm they are doing to their citizens by refusing to cooperate with federal law enforcement officers,” he said.

In 2015, Garcia Zarate served time in a federal prison for his latest illegal entry into the U.S. Officials then sent him to San Francisco, where he was wanted on an old warrant for a drug charge.

But local prosecutors decided they didn’t want to pursue the case. Rather than send Garcia Zarate back to the federal government for deportation, however, he was released into the community under the local sanctuary policy.

Less than four months later, he and Steinle’s paths would collide on the waterfront.

Mr. Trump, who just days earlier announced his presidential bid vowing a tough-on-immigration approach, said Steinle’s killing was a “senseless and totally preventable act of violence.”

“The American people deserve a wall,” he said.

More than two years later, Mr. Trump is fighting Congress for money to begin construction of his wall, and San Francisco’s sanctuary policy remains largely intact.

Indeed, the city is in court battling the Trump administration’s efforts to crack down on sanctuaries, objecting to the president’s plans to try to withhold federal money from jurisdictions that won’t communicate with deportation officers.

While a few jurisdictions have revoked sanctuary policies in the wake of Mr. Trump’s election victory, more have declared themselves sanctuaries and vowed to resist the administration.