The Palmetto State Armory M4 assault rifle used in the shooting that killed an NYPD cop and three civilians is .223-caliber and is equipped with a black scope, a handguard, and a shoulder sling. The AR-style gun retails for around $500 and was equipped with a high-capacity magazine.

The 27-year-old man who shot and killed four people at a Midtown office building on Monday carried a note in his pocket claiming he suffered from CTE, asked that his brain be studied and made references to the NFL.

Police are investigating a reported active shooter at a Midtown Manhattan high-rise. Here’s what we know about the suspect involved in the incident near Park Avenue.