UnitedHealthcare CEO killing: Luigi Mangione's gun matches shell casings from crime scene, NYPD says

The ghost gun allegedly in Luigi Mangione's possession when he was arrested in Pennsylvania has been matched to three shell casings recovered at the scene of Brian Thompson's murder in New York City, NYPD Police Commissioner Jessica Tisch said Wednesday.

Fingerprints recovered from a water bottle and a Kind bar near the crime scene have also been matched to Mangione, she said.

Mangione, a 26-year-old Ivy League graduate, is accused of gunning down Thompson, the UnitedHealthcare CEO, outside a Midtown Manhattan hotel on Dec. 4.

Written on the shell casings were the words "deny," "defend" and "depose," according to police sources.

Mangione was apprehended in Altoona, Pennsylvania, on Monday after nearly one week on the run.

Mangione allegedly had a spiral notebook detailing plans about how to eventually kill the CEO, according to law enforcement officials.

One passage allegedly said, "What do you do? You whack the CEO at the annual parasitic bean-counter convention," the officials said.

The writings said using explosives in the attack could "risk innocents," according to the officials.

Detectives are still examining Mangione's writings but are considering the contents of the notebook to represent a confession, sources said.

Investigators have started interviewing members of Mangione's family, according to sources.

A judge in Pennsylvania ordered Mangione held without bail on Tuesday. In Pennsylvania he faces charges including allegedly possessing an untraceable ghost gun.

Mangione plans to challenge his extradition to New York, where he faces charges including second-degree murder.

"He has constitutional rights and that's what he's doing" in challenging the interstate transfer, defense attorney Thomas Dickey told reporters on Tuesday.

Mangione is "taking it as well as he can," Dickey added.

The Manhattan District Attorney's Office said it will seek a governor's warrant to try to force Mangione's extradition. New York Gov. Kathy Hochul said in a statement that she'll sign a request for the governor's warrant "to ensure this individual is tried and held accountable."

Mangione's attorney told ABC News' "Good Morning America" on Wednesday that anyone speculating on the case should take the potential evidence "in its entirety," not taking pieces of writing or other evidence "out of context."

"People put out certain things, parts of different things," he said. "I think any lawyer involved in this situation would want to see it all."

Mangione plans to plead not guilty to the charges in Pennsylvania, Dickey said. Dickey said he anticipates Mangione would also plead not guilty to the second-degree murder charge in New York.

ABC News' Sasha Pezenik, Mark Crudele, Luke Barr, Peter Charalambous and Josh Margolin contributed to this report.