NEW YORK (PIX11) — NYPD detectives found a loaded, AR-15 assault rifle under the mattress of alleged cop killer Lashawn McNeil, officials said Monday.

The discovery was made Saturday after detectives secured a search warrant for the Harlem apartment where McNeil had been staying with his mother and brother on West 135th Street. The NYPD tweeted a photo of the assault weapon late Monday afternoon.

McNeil, a 47-year-old ex-con, was accused of ambushing the two officers when they went to speak to him in a back bedroom inside the apartment. Officials said McNeil died Monday of injuries sustained during the shootout.

NYPD Chief of Detectives James Essig said McNeil used a Glock 45 to shoot the young officers, Jason Rivera, who was a 22-year-old rookie, and Wilbert Mora, a 27-year-old officer on the job four years.

Both officers were shot in the head. Rivera was pronounced dead shortly after the shooting. Mora suffered a significant brain injury and he was transferred Sunday evening to NYU Langone Medical Center.