The glittering skyscraper where hundreds work became the scene of New York City’s deadliest gun attack in 25 years when a man entered the lobby and started shooting.

The deadly Park Avenue shooting happened at one of the the most exclusive addresses in Midtown Manhattan — the “Blackstone Building,” which is home to financial giant Blackstone, the NFL and

The alarm reached Jon Gray on the 44th floor, in his C-suite office high above Park Avenue: An active shooter was in the building.In the lobby below, a gunman had opened fire, killing a uniformed police officer and two others who worked in the office tower.

The office building, located at 345 Park Avenue, houses some of the country’s top financial firms, including Blackstone, and the National Football League. According to the company's website, Blackstone stands as the world’s largest alternative asset manager, managing over $1 trillion in assets under management.

Shane Tamura, the Park Ave. gunman who killed an NYPD officer and three others before taking his own life, left behind a suicide note saying he suffered from CTE, a brain injury often linked to playing football, police sources said Tuesday.

NEW YORK -- An active investigation is underway after a New York City police officer and civilian were shot in Midtown, Manhattan, according to police sources. The shooting happened outside 345 Park Ave., which is the building that contains Blackstone and the NFL headquarters.

Five people were shot and four killed by the 27-year-old gunman Shane Tamura, when he opened fire inside an office building at 345 Park Ave. on Monday evening.

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.

Nearly 24 hours after a deadly Midtown mass shooting that left four people dead, including an off-duty police officer, detectives from the NYPD Crime Scene Unit combed through the plaza outside 345 Park Ave.