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

An off-duty New York City police officer and an executive at an investment firm were among four people killed by a gunman at a Manhattan office tower.

Multiple casualties as gunman opened fire in the lobby of the midtown Manhattan building on Park Avenue that is home to the Blackstone investment firm and other corporations before moving to other floors.

An NFL employe was seriously injured on Monday in a shooting at a New York City office tower that killed four people, commissioner Roger Goodell said in a statement.

New York City Police Department officer Didarul Islam was shot and killed at a Manhattan office building where a gunman killed three other people. Mayor Eric Adams ordered flags on all city buildings to be lowered to half-staff to honor victims of the shooting.

Arriving officers found 11 people shot including a 27-year-old man pronounced dead at the scene, Schierbaum told reporters during a late-morning news conference. The other 10 victims included an 18-year-old man who was taken to a hospital in critical condition.

Mobile police have released the identity of the man killed in a shooting Friday afternoon. At approximately 2:46 p.m., officers responded to the 400 block of Earle Street in reference to a shooting, according to a previous release. Upon arrival, officers discovered Cameron Madison, 24, with a gunshot wound.

NYC Mayor Eric Adams said the gunman appeared to be targeting the NFL over CTE, a degenerative brain disease.

NYC is mourning NYPD Officer Didarul Islam and three other people killed in Monday's shooting inside an office building at 345 Park Avenue.

Shane Tamura, the suspect in the deadly shooting in Midtown Manhattan, carried a note in his pocket claiming he suffered from CTE and also made references to the NFL, police sources told ABC News.