The gunman who killed an NYPD officer and three other people in a Midtown building on Monday evening was carrying a note that offered clues into his possible motive, sources told The Post.

Wearing body armor and wielding an assault-style rifle, the alleged gunman burst into a Midtown Manhattan office building shooting victims at random.

The police said the man accused of the killings referred to chronic traumatic encephalopathy, which is caused by physical trauma, and asked that his brain be studied.

Companies including Blackstone, NFL and KPMG close offices and provide employee support following Midtown Manhattan shooting tragedy inside 345 Park Avenue.

The gunman who killed three civilians and an NYPD officer inside a Manhattan office building Monday evening left a note and was apparently targeting NFL offices.

New York City mayor Eric Adams said Tuesday that the 27-year-old gunman who killed four people and injured another in a shooting at 345 Park Avenue on Monday was trying to target the NFL's headquarters, which are located in that building, but inadvertently took the wrong elevator.

Multiple people were killed in a shooting Monday in a corporate office building in Midtown Manhattan, according to media reports. The New York Police Department posted on X that the scene “has been contained and the lone shooter is dead.”

Julia Hyman, an employee at Rudin Management, was working on the 33rd floor of 345 Park Avenue when Shane Devon Tamura, 27, opened fire and killed her shortly before he turned his weapon on himself, a law enforcement source confirmed to PEOPLE.

A police officer and two civilians were shot at a Midtown Manhattan office building that houses The Blackstone Group and NFL headquarters in what officials were calling an active shooter investigation.