UPDATED: Five people were killed, including at least one off-duty New York Police Department officer, after a gunman stormed a Midtown Manhattan skyscraper early Monday evening and opened fire before turning the gun on himself, authorities said.

Among the fatalities was a female officer.

According to international news reports, the suspect entered the 44-story building, which houses several high-profile corporations, around 6:30 p.m.

The tower includes offices of JPMorgan, KPMG, Deutsche Bank, Bank of America, the National Football League’s headquarters, and investment firm Blackstone, reportedly the building’s largest tenant.

Armed with a rifle, the gunman began firing, fatally striking several individuals, including a responding police officer. Several civilians were also injured and rushed to nearby hospitals, officials said.

Reports say the gunman, believed to be a 27-year-old from Las Vegas, entered the building and, while in the lobby, fired at a police officer who was working private security at the time and was shot and killed, along with at least two others.

Police sources said the suspect, who reportedly held an expired private investigator license, killed himself on the 33rd floor of the building. It remains unclear how he gained elevator access to reach that floor.

At least six people were shot in Midtown Manhattan. Three civilians were hospitalized, and their conditions remain uncertain.

The number of casualties is still developing.

As of late Monday, police were still clearing floors of the building, and occupants were instructed to barricade themselves as a precaution. The entire 41st floor has been sealed off, according to reports.

Agence France-Presse (AFP) reported Monday that New York City Police Commissioner Jessica Tisch said the gunman who had been on the loose in the busy Midtown district had been “neutralized.”

The FBI is on the scene assisting with the response. Injured victims are currently receiving medical attention.

This is a developing story.

UPDATE: AFP confirmed that five people, including a police officer and the suspected gunman, were dead following a shooting in central Manhattan on Monday, U.S. media reported.

"Four people, including a New York Police Department officer, were killed in today's shooting in Midtown Manhattan," a law enforcement source told CNN, adding the suspected gunman died from what is believed to be a "self-inflicted injury."