Gunman Kills 4, Including Police Officer, in Midtown Manhattan Shooting

In the United States, after 4 people died in a deadly shooting in Midtown Manhattan on Monday, investigators have been sent to Las Vegas, where the gunman, who also shot himself, lived, to learn more about the possible motive.

Investigators, who recovered a handwritten note, say there were no links to terrorism or criminal organizations found.

The note, found on the body of 27-year-old Shane Devon Tamura, said, "Study my brain. I'm sorry."

It also claimed he suffered from CTE, a brain disease triggered by head injuries, and blamed the NFL for his condition, despite having never played football professionally.

Tamura, who had driven from Las Vegas, entered the lobby of a building housing the National Football League headquarters and major financial firms at around 6.30 PM on Monday with an assault-style rifle.

The victims included an off-duty New York City police officer, who was described as a "New York Hero."

The incident marks the city's deadliest mass shooting in over two decades.