12:23 PM PT -- The gun allegedly found in Luigi Mangione's possession when he was arrested Monday has been forensically tested ... and New York Police confirm the "ghost gun" has been matched to shell casings found at the scene of UnitedHealthcare CEO Brian Thompson's murder.

NYPD Commissioner Jessica Tisch said the gun is being processed at the NYPD crime lab ... and said the gun matches "the three shell casings" found in midtown Manhattan a week ago.

As TMZ reported ... the words "deny," "defend" and "depose" were engraved in shell casings found at the scene.

Police say Luigi Mangione's fingerprints match prints found at the scene of Brian Thompson's murder last week in New York City.

CNN reports two law enforcement officials confirmed that information Wednesday morning.

It was previously reported a partial fingerprint was lifted from a water bottle, possibly bought at a Starbucks down the street from the shooting scene moments before the crime. It's unclear if that's the print to which police are referring.

Mangione was arrested Monday in an Altoona, PA McDonald's after a 5-day manhunt. He's fighting extradition to New York, where authorities have charged him with murder. Police say he had a handwritten manifesto in his possession.

United Healthcare CEO Brian Thompson was fatally shot outside the New York Hilton Midtown hotel last week. A masked gunman approached him from behind and shot him multiple times before escaping on an e-bike.

Mangione was briefly seen outside the Pennsylvania courthouse before his extradition hearing Tuesday, where he shouted at reporters ... and was then rushed into the building by sheriff's deputies. His attorney Tom Dickey said yesterday he's seen no evidence connecting his client to the crime.

We reached out to Dickey regarding the fingerprints report, but haven't heard back from him.