A gunman wielding an assault-style rifle has stormed a skyscraper in the heart of New York, killing four people before shooting himself dead.

The attacker, 27-year-old Shane Tamura of Las Vegas, walked into an office block on Manhattan's Park Avenue on Monday evening and opened fire in the lobby before taking a lift to the 33rd floor to continue his spree.

Four people were killed including New York City police officer Didarul Islam, 36, who was working security at the time. The three other victims have yet to be identified.

The gunman appears to have driven across the US from Las Vegas. Police say he was known to have mental health issues, and they are searching for a motive.

The shooting sent pedestrians running for cover as police warned of an active gunman around 18:30 ET (22:30 GMT). Those inside the building cowered as gunfire echoed - some even piling couches and chairs against doors as they hid in offices.

The skyscraper is home to the National Football League (NFL), as well as financial giants like Blackstone and KPMG and other major firms.

New York City Police Commissioner Jessica Tisch said the suspect appeared to have acted alone. She told a press conference that a fifth victim was shot and was in "critical but stable condition".

The slain police officer Didarul Islam, a Bangladeshi immigrant and father of two with a third child on the way, had served in the New York Police Department for three and a half years.

Local officials and authorities celebrated Islam, saying he died a "hero."

Tisch said: "He was doing the job that we asked him to do. He made the ultimate sacrifice."

After shooting Islam as he entered the building, Tamura then aimedat a woman who hid for cover behind a pillar. He proceeded through the lobby, "spraying" it with gunfire, hitting multiple people - including a security guard hiding behind a desk and another man nearby.

As he waited for an elevator, a woman stepped off, but the gunman spared her and let her pass, Commissioner Tisch said.

He made his way to the 33rd floor, killing one person before walking down a hallway and shooting himself in the chest, she added at a Monday night news conference.

Police searched the suspect's vehicle, which he had double-parked out front, and found multiple magazine rounds and a revolver. The vehicle was registered in Nevada to Tamura, who was a registered gun owner in the state.

The incident brought parts of Midtown Manhattan and public transportation to a halt. A BBC journalist at the scene reported seeing scores of police vehicles and at least one person with a bloodied chest being taken away on a stretcher.

Bystanders reported hearing what sounded like gunshots and police told those in the area, including the BBC journalist, to shelter in nearby buildings.

Police worked floor-by-floor to clear the building, an effort that took hours.

Mama Bouhenni, working at the nearby Sip & Scoop coffee shop as the incident unfolded, told the BBC she saw dozens of people from surrounding buildings being evacuated.

They held their hands above their heads, she said.

Ms Bouhenni said she recognised many of them as customers. "This is so terrifying for them, I hope they can go home safe."