STONEHAM, Mass. – A man pleaded not guilty Wednesday to charges he fatally shot his firefighter friend while they were drinking alcohol and playing with a handgun.

Patrick Riccardi-O’Connor, 23, was arraigned Wednesday in Woburn District Court on charges of involuntary manslaughter and related crimes in the death of David Atherton, 24, a Stoneham firefighter and National Guardsman.

Police found Atherton dead Tuesday night inside his Stoneham home.

Middlesex District Attorney Marian Ryan said a preliminary investigation shows that Riccardi-O’Connor and Atherton were drinking with a third man. Riccardi-O’Connor and Atherton — childhood friends — began passing a gun back-and-forth and pointing it at each other.

“It appears the fatal shooting may have been an unintended consequence of this behaviour,” Ryan and Stoneham Police Chief James McIntyre said in a statement.

Police said Riccardi-O’Connor originally told them Atherton had shot himself but later admitted shooting Atherton while the two were “fooling around with the gun.” Riccardi-O’Connor told police he didn’t think the gun was loaded.

Riccardi-O’Connor’s attorney didn’t immediately return a message seeking comment.

Atherton received a single gunshot wound to the chest. The gun was registered to Riccardi-O’Brien. Both men were military police officers in the Massachusetts Army National Guard.

Judge Michael Brooks set bail at $25,000 cash. The next court hearing in the case is scheduled for Aug. 3.