Chaos erupted in the heart of Manhattan Monday evening as a heavily armed gunman stormed a high-rise office building on Park Avenue, killing four people — including a New York City police officer — before taking his own life.

The attacker, identified as 27-year-old Shane Tamura of Las Vegas, entered the skyscraper around 6:30 p.m. ET wielding an assault-style rifle. He opened fire in the building’s lobby, fatally shooting Officer Didarul Islam, a 36-year-old security detail and NYPD officer, before continuing his deadly rampage on the 33rd floor.

The building, which houses the headquarters of major corporations including the National Football League (NFL), Blackstone, and KPMG, was placed on lockdown as panicked workers barricaded themselves inside offices using chairs and couches.

Three additional victims, whose identities have not been released, were killed in the attack. A fifth person is in critical but stable condition.

New York Police Commissioner Jessica Tisch confirmed Tamura acted alone and had traveled cross-country from Nevada. Authorities say he was a registered gun owner with a documented history of mental health issues, though no clear motive has been established.

“He was doing the job we asked him to do. He made the ultimate sacrifice,” Commissioner Tisch said of Officer Islam, a Bangladeshi immigrant and father of two, with a third child on the way. His death has drawn an outpouring of grief and praise from city leaders and community members who hailed him as a hero.

According to police, after gunning down Islam at the building’s entrance, Tamura sprayed the lobby with bullets, striking a security guard behind a desk and another man nearby. He spared a woman who exited the elevator as he waited, before heading to the 33rd floor where he killed another person and then fatally shot himself.

Police recovered multiple magazines and a revolver from Tamura’s double-parked vehicle, which was registered in Nevada. The incident brought Midtown Manhattan to a standstill, halting traffic and public transportation as emergency services secured the area.

A BBC correspondent at the scene reported intense police presence, with one wounded victim seen being rushed away on a stretcher.

The mass shooting has once again reignited national debate over gun control and mental health in the United States.