ALSO SEE: ICE Likely Violated First Amendment by Issuing Warning to Critic, Judge Rules

Federal agents showed up at the Rochester, N.Y., home of a man who had sent a caustic email to an immigration official and threatened him with prosecution.

A federal judge this week ruled that the Trump administration appeared to have violated the First Amendment rights of a Rochester, N.Y., man by threatening him with prosecution after he sent a scathing email to a top immigration official.

Federal agents had delivered a written warning notice in June to the home of the man, David Streever, informing him that he “may be in violation of federal law” and directing him to stop criticizing the acting director of the Immigration and Customs Enforcement agency.

Judge Rudolph Contreras of the U.S. District Court for the District of Columbia said in a decision issued Tuesday that the warning probably amounted to unconstitutional coercion of protected political speech.

“Americans have long voiced their displeasure with their government,” the ruling began.

“Today is no different,” the judge later wrote, adding, “What is different is how the government responded in this case.”

The decision temporarily barred the government from acting on the warning and from issuing similar notices to Mr. Streever as his lawsuit, which was filed against the federal government by free-speech advocates, continues. The government has 14 days to appeal.

Legal experts said the ruling underscored the broad protection that the First Amendment affords to even harsh criticism of government officials.

Mr. Streever had emailed the acting director of ICE, Todd Lyons, in January after the fatal shootings by federal officers of two demonstrators in Minneapolis, Renee Good and Alex Pretti, who were protesting an immigration crackdown.

The email said that Mr. Lyons would be haunted by the shootings and “will never know peace” and likened him to Reinhard Heydrich, the Nazi security chief who helped orchestrate the Holocaust.

Michael Moreland, a law professor at Villanova University, said that the ruling is consistent with recent First Amendment cases and could help other people who have received similar warnings from the government.

“You can have language in politics and public debate that can be pretty strident,” Mr. Moreland said. “But that doesn’t mean it’s a threat that the government can then launch its investigatory apparatus against.”

The Department of Homeland Security, the parent agency of ICE, issued a statement on the ruling that mirrored an earlier defense of its actions. The department did not say whether it would appeal the decision.

“Any allegation that D.H.S. and its components are attempting to ‘squash’ free speech is categorically false,” the statement read. “ICE investigates all credible threats toward its employees and officers, including threats to the ICE director.”

Judge Contreras noted in his ruling that the government appeared to have conceded that Mr. Streever’s email was protected speech and that its investigation into him had been closed.

The Foundation for Individual Rights and Expression, a free-speech watchdog group, had sued the Trump administration in July on behalf of Mr. Streever after federal agents visited his home and a hotel where he was staying. The agents had gone to his home with a document that said he may have violated a law against threatening a federal law enforcement officer and warned that he could face prosecution if he continued.

Adam Steinbaugh, a lawyer at the Foundation for Individual Rights and Expression, disputed the assertion that the email contained a threat and praised the court ruling as a victory.

“ICE knowingly pursued an American citizen for criticizing his government — speech that is at the heart of the First Amendment,” Mr. Steinbaugh said. “Ominous letters warning people their speech might land them in prison are meant to coerce people into silence, and that’s not how the First Amendment works.”

Perry Grossman, a supervising attorney at the New York Civil Liberties Union, said that the government’s use of the warning was significant because of its explicit threat of criminal prosecution.

“They just put in writing that if he continues, they’re going to drop the sword of Damocles on his head and put him in prison,” Mr. Grossman said. “You can’t imagine a more intimidating, censorious threat than that.”

Mr. Streever said in a statement issued by his lawyer that he hoped the ruling would serve as a reminder of First Amendment protections.

“I’m deeply relieved that the court affirmed what I knew all along: that I am permitted to speak my conscience in the United States of America, including when I criticize government officials,” he said.