A US man has been sentenced to 13 months in prison for making over 12,000 harassing and threatening calls to the offices of congressional lawmakers.

Ade Salim Lilly, 35, pleaded guilty in May, admitting to the court that he made the calls between February 2022 and November 2023.

He tried to contact some 54 members of Congress, both at their district offices and at their offices in Washington DC. He threatened to kill at least one staff member.

The federal judge overseeing Lilly’s case also sentenced him to three years of supervised release after he completes his prison term.

Over the course of two days in February 2023, Lilly called one lawmaker more than 500 times.

Most of his interactions were with congressional staff or interns, officials said.

In one call, he threatened a staff member: “I will kill you, I am going to run you over, I will kill you with a bomb or grenade.”

Lilly, a resident of Queens, New York, was arrested in November 2023 after moving to Puerto Rico.

Capitol Police Chief J Thomas Manger testified to the court last year that the harassment campaign coincided with a 400% increase in threats against lawmakers in Congress over the past six years.

Lilly pleaded guilty in May to interstate communications with a threat to kidnap or injure and making repeated telephone calls.

Prosecutors had sought a sentence of 18 months, writing in a sentencing memo that the current election cycle creates a risk that such threats could become “normalised”.

On Tuesday, Chief Manger, whose force protects members of Congress and the Capitol, released a statement saying: “We will enforce the law anytime someone crosses the line from free speech to harassment or threats.”

Lilly’s motive and ideology are unclear, according to public records.

Addressing the court during his sentencing on Tuesday, he said: “My intent was always to do what’s best for our future generations.”

The judge noted the recent uptick in violent attacks against politicians, citing Donald Trump’s attempted assassination and the hammer attack against Nancy Pelosi’s husband.

“We’ve got a real problem on our hands,” he said before passing sentence.