Mayor Eric Adams told MSNBC the gunman referenced CTE, a disease linked to head trauma, in a note before killing four people at 345 Park Ave., which houses the NFL.

The gunman who opened fire in midtown Manhattan on Monday, killing four people before turning the gun on himself, left a note mentioning “CTE” several times, officials familiar with the investigation told NBC News.

The New York City office building that was the site of a mass shooting Monday had significant security measures in place.

Four people, including an off-duty NYPD officer, were killed Monday evening in a shooting at a Manhattan high-rise corporate building.

A gunman opened fire in a prominent Manhattan office building on Monday evening, killing four people and seriously injuring a fifth before dying from a self-inflicted gunshot wound, New York City officials said.

New York City police detectives searched a studio apartment in Las Vegas that belonged to the suspected gunman in Monday's mass shooting at a Midtown Manhattan.

In the aftermath of New York’s deadliest mass shooting since 2000, authorities are investigating the motives and background of the man who opened fire in a midtown Manhattan office skyscraper, killing four people and then himself.