He said Mr. Mangione had pleaded not guilty to the charges brought against him in Pennsylvania, including the firearms charges.
He was charged in New York with second-degree murder in connection with Mr. Thompson’s murder. The 50-year-old UnitedHealthcare executive was shot and killed by a masked gunman outside a Manhattan hotel on Dec. 4 in what authorities said was a targeted attack.
Mr. Mangione is currently in a state prison in Pennsylvania, where he is fighting extradition to New York to face murder charges. The legal battle over his extradition could potentially take more than a month, officials said.
But legal experts told the BBC that his efforts to challenge his extradition to New York were unlikely to succeed. However, they could give his defense a look at the state’s evidence against him.
“I don’t even know if it’s him,” his lawyer, Mr. Dickey, said in a recent interview with US media outlet NewsNation, referring to images of Mr. Thompson’s killer.
“We’re going to test those waters and give the government an opportunity to present some evidence,” he said.
Experts say if he is extradited to New York to face murder charges, Mr Mangione and his legal team will face an uphill battle as they try to defend themselves.
Mitchell Epner, a New York lawyer and former prosecutor, told the BBC that there were broadly two approaches Mr Mangione could take if he pleaded not guilty to Mr Thompson’s murder.
“Defense number one is, ‘It wasn’t me,’ and defense number two is, ‘It was me, but I shouldn’t be punished’ because of X,” he said.
According to the New York Police Department, Mr. Mangione was found with a gun that appeared to be an assassination device, a silencer and a fake ID, along with three handwritten pages that they believed showed a potential motive.
Mr Epner said the public evidence so far meant denial of responsibility was “out the window”.
Another New York criminal defense lawyer and professor, Dmitry Shakhnevich, said Mr. Mangione’s lawyer could theoretically also argue that his “mental state” impairment makes him unfit to stand trial.
“If the judge makes a determination that he doesn’t understand or doesn’t understand what’s going on in court, then basically the case won’t go forward,” he said.
“He will be in the dormitory for a period of time until he is deemed fit, which may never be.”
That defense, Mr. Shakhnevich added, is different from an insanity plea, in which his lawyers can argue that “he is not responsible for his actions because of some mental defect.”
“It might as well find him not guilty because you don’t satisfy an element of the offense,” he said. “But again, he doesn’t walk free. He would be institutionalized for a period of time, assuming the defense is successful.”
