NYC shooting victims Didarul Islam, Wesley LePatner, Aland Etienne, and Julia Hyman were killed by a gunman inside Manhattan's 345 Park Ave.

Four people were killed and a fifth was injured in a mass shooting at a Midtown Manhattan office building on Monday.

A New York City police officer who was among the four people shot and killed by a gunman during a rampage at a Manhattan office tower will be laid to rest Thursday. Officer Didarul Islam, 36, was fatally shot Monday in a midtown building housing the National Football League’s headquarters.

The attack on a seemingly secure building has rattled workers and prompted managers to examine whether they are adequately protected.

The man who opened fire in a Midtown Manhattan office building, killing four people and wounding a fifth, appeared to be targeting the NFL headquarters and had a note claiming he suffered from CTE, police sources told ABC News.

The gunman who killed four people in a midtown Manhattan office building left a suicide note indicating that he was targeting the NFL, police said.

Four people were killed, including an off-duty New York City police officer, when a gunman opened fire in an office tower before fatally shooting himself, officials said.

The man who killed four people in the New York City skyscraper that is home to NFL headquarters carried a note blaming the league for his mental health problems

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.