San Francisco is one of dozens of US cities that refuses to cooperate with federal immigration agencies.
Garcia Zarate had been deported from the United States back to Mexico five times.
The father of a woman who was shot and killed on a San Francisco pier said he was shocked when a jury acquitted a Mexican immigrant accused of murder in the case.
"There was a tremendous amount of misinformation that was spread about this case from day one", San Francisco Public Defender Jeff Adachi told reporters outside the courtroom.
"The Department of Justice will continue to ensure that all jurisdictions place the safety and security of their communities above the convenience of criminal aliens", Sessions said in Thursday's statement.
In returning its verdict on the sixth day of deliberations, the San Francisco Superior Court jury found Jose Ines Garcia Zarate guilty of a single lesser charge of being a felon in possession of a gun.
Defense attorney Matt Gonzalez said Garcia Zarate found the gun at the pier.
Defense attorney Matt Gonzalez added: "The verdict does not diminish in any way the terrible tragedy" the Steinle family went through.
They spoke to the Chronicle in an exclusive interview that they said would be their last public comments.
Even if Garcia Zarate had received a sentence of 100 years, the father said, "it doesn't solve anything, it doesn't help anything".More news: WhatsApp may add some new features in the app for iOS
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The case became a lightning rod for Trump and others in the push to halt illegal immigration and penalize so-called sanctuary cities, including San Francisco. The bullet ricocheted on the pier's concrete walkway before it struck Steinle.
Zarate faced a charge of second-degree murder, but jurors were instructed to consider first-degree murder and involuntary manslaughter.
Prosecutors said Garcia Zarate immediately tried to cover his tracks by throwing the gun into the San Francisco Bay, then fleeing the scene. However, he was found guilty of possession of a firearm.
Defense attorney Francisco Ugarte said Thursday the death of Kate Steinle was an "incomprehensible tragedy", but the ruling was a vindication for immigrants.
Garcia Zarate did not deny shooting Steinle in 2015 and said it was an accident. He had been deported five times before.
Jurors have found a Mexican man not guilty of murder in the killing of a woman on a San Francisco pier in a case that touched off a national immigration debate. But Zarate's defense attorney contended the shooting was accidental.
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.
At trial, Zarate admitted to firing the shot that killed Steinle, 32, but contended that he had picked up a gun underneath a bench on the San Francisco pier and accidentally fired it July 1, 2015.
But he told jurors that Garcia Zarate had no motivation to kill Steinle and as terrible as her death was, "nothing you do is going to fix that". Word came hours after they asked to see the gun used in the shooting. San Francisco's "sanctuary city" law bars local officials from cooperating with federal attempts to deport those in the country illegally, and the city had released Garcia Zarate from custody despite a federal detention request.
President Donald Trump's son, Donald Trump Jr. responded to the verdict and tweeted out saying: "What a disgrace".