A gunman who killed four people when he stormed a skyscraper in the heart of New York on Monday evening left a note that appeared to blame the National Football League (NFL) for a brain injury, the city's Mayor Eric Adams says.

The attacker, 27-year-old Shane Tamura of Las Vegas, shot himself dead after opening fire in a building where the American football league has its headquarters, but went to a different part of the building after taking the wrong lift.

The gunman was carrying a note in which he blamed CTE, a brain disease triggered by head trauma, for his mental illness, Adams said.

Tamura played football as a teenager but did not play in the NFL, ex-teammates have told US media.

New York City police officer Didarul Islam, 36 - who was working as a security guard at the building - was among those killed.

Another of the victims was an employee of finance giant Blackstone, who was named by her company as Wesley LePatner.

Two male civilians were also killed. An NFL employee was also "seriously injured" in the attack, the league's commissioner Roger Goodell wrote in a message to staff.

A statement from US President Donald Trump, who has been visiting Scotland, said he had been briefed on the "tragic shooting".

The statement said Trump trusted US law enforcement agencies to "get to the bottom of why this crazed lunatic committed such a senseless act of violence".

Asked about a possible motive on the part of Tamura, Eric Adams told CBS: "[He] did have a note on him. The note alluded to that he felt he had CTE, a known brain injury for those who participate in contact sports.

"He appeared to have blamed the NFL for his injury."

CTE, or chronic traumatic encephalopathy, is a brain disease linked with repeated blows to the head.

Tamura's alleged note said "study my brain please", according to a senior official who spoke to ABC News. Tamura is said to have then written: "I'm sorry."

The alleged attacker was an American football player during his time at high school in California, shocked ex-teammates told NBC News.

One classmate remembered Tamura as a jokey personality, and a former coach described him as a talented and hard-working running back who made "a great player".

The gunman appears to have driven across the US from Las Vegas to New York, and used an assault-style rifle during the attack at 345 Park Avenue.

After opening fire in the building's lobby, Tamura is believed to have entered a lift to the 33rd floor of the skyscraper and continued to fire.

Mayor Adams said a preliminary investigation showed that the gunman mistakenly went to the office of Rudin Management, which owns the building.

Tamura later turned his gun on himself.

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.

One woman, Nekeisha Lewis, said she was eating dinner with friends on the plaza when she heard gunfire. "It felt like you were in a warzone almost," she told NBC News.

Ms Lewis said she saw an injured man run out of the building, and described the incident as "literally the most scary situation I've ever been in".

City officials paid tribute to Islam, who migrated to the US from Bangladesh, had two children and was expecting a third with his pregnant wife.

"He was doing the job that we asked him to do," said New York Police Commissioner Jennifer Tisch. "He put himself in harm's way. He made the ultimate sacrifice."

Tisch continued: "He died as he lived - a hero."

Members of Islam's Bangladeshi community told the New York Times how Islam had migrated to the US and worked as security for a school before becoming a policeman. They said their friend had been an active member of his mosque who had mentored young men in the area.

Blackstone employee Wesley LePatner was described by her company as "brilliant, passionate, warm, generous, and deeply respected within our firm and beyond.

"She embodied the best of Blackstone."

Details about the two other victims, or the NFL employee who was hurt in the shooting, have not yet been made public. (BBC)

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