Mangione's guilty plea could now throw a wrench into his New York state case.

With the trial scheduled to begin in September, he faces eight state charges including second degree murder, possession of a forged instrument (for alleged fake driver's licence) and six counts of criminal possession of a weapon.

On the steps of the court on Friday, his lawyers argued that because he had already pleaded guilty to federal charges, the state case ran afoul of double jeopardy – a constitutional protection that prevents an individual from being prosecuted twice for the same crime.

"New York law does not permit a person to be prosecuted and punished twice for the exact same crime," his lawyer Karen Friedman Agnifilo said.

She also said that, in making a guilty plea in federal court, Mangione has accepted "full responsibility" for the death of Brian Thompson.

But the Manhattan District Attorney's office says it is prepared to go ahead with the state prosecution.

"We are encouraged that Mr Thompson's family received a measure of accountability today," the statement said.

"While federal sentencing is pending, we are prepared to litigate the defence motions. The Manhattan DA's Office remains committed to seeking justice for Mr Thompson and his family."

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