It began with a hate-filled call to Representative Ilhan Omar’s Washington office in March, officials said.
“Do you work for the Muslim Brotherhood?” Patrick W. Carlineo Jr. asked a member of Ms. Omar’s staff on that call, according to a criminal complaint. “Why are you working for her, she’s a [expletive] terrorist. Somebody ought to put a bullet in her skull. Back in the day, our forefathers would have put a bullet in her [expletive].”
“I’ll put a bullet in her [expletive] skull,” the staff member also recalled Mr. Carlineo, 55, saying, according to the complaint.
On Monday, Mr. Carlineo, of Addison, N.Y., outside of Buffalo, pleaded guilty to threatening to kill Ms. Omar, a Minnesota Democrat and one of the first Muslim women elected to Congress, and to possessing guns illegally.
Ms. Omar responded to Mr. Carlineo’s guilty plea in Federal District Court in Buffalo by urging leniency when he is sentenced.
In a letter she posted on Twitter on Tuesday, Ms. Omar addressed the judge who will sentence him, asking “for a system of compassion to be applied.”
“Threatening assassination of a public official in our country is dangerous to both the individual and our republic,” Ms. Omar wrote, describing the crimes that Mr. Carlineo admitted to as “grave” and adding that “threats of political violence and hate speech” were “not unique” to him.
“They are an increasing feature in our public sphere,” Ms. Omar, a Somali refugee whose family received asylum in the United States when she was a teenager, added. “We will not defeat it with anger and exclusion. We will defeat it with compassion.”
A member of her party’s progressive wing, Ms. Omar — the first woman to wear a hijab on the House floor — has been denounced in some quarters, including by some Democrats, for embracing the boycott-Israel movement and criticizing supporters of the Jewish state.
In an interview with the F.B.I. at his home about a week after he called Ms. Omar’s office, Mr. Carlineo described himself as a patriot who loved Mr. Trump and hated “radical Muslims in our government,” the criminal complaint says.
He also told an F.B.I. agent that he believed Ms. Omar supported Islamic militant groups, such as Hamas and the Muslim Brotherhood, and that her election to Congress was illegitimate.
In the interview, the complaint says, he acknowledged calling Ms. Omar’s office and said that he knew why the F.B.I., which investigated the matter with the United States Capitol Police, was interested in talking to him.
Shown an email containing the text of the call, he initially denied that the words were his, but later “went on to say that he may have said something” similar, the complaint says. (He had left his name and contact information with Ms. Omar’s staff, the complaint says.)
The F.B.I. agent who signed the complaint said that Mr. Carlineo had threatened to kill Ms. Omar “while she was engaged in the performance of her official duties.”
About a week later, the authorities found a loaded .45-caliber handgun, three rifles, two shotguns and hundreds of rounds of ammunition at Mr. Carlineo’s home, prosecutors said. Because of a 1998 felony conviction, he was legally prohibited from owning a firearm.
Mr. Carlineo is scheduled to be sentenced on Feb. 14. He faces up to 10 years in prison. He remains free until sentencing under conditions that include a curfew and a restriction against traveling outside of western New York.
Barbara Burns, a spokeswoman for the United States Attorney’s Office in Buffalo, declined to comment on Ms. Omar’s sentencing request.
Susan C. Beachy contributed research.