The 27-year-old Nevada man left a note at the scene voicing anger that his mental illness may be linked to his having played football, officials familiar with the investigation told NBC News.

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.