The gunman who killed four people and then himself inside a Manhattan office building on Monday had arrived in the city just hours before, after driving cross country for days from his home in Nevada, Police Commissioner Jessica Tisch said on Monday night.

Ms. Tisch identified the man as Shane Tamura, 27, and said it was unclear what had motivated him to drive from Las Vegas to unleash mayhem in the heart of Manhattan at the end of the work day.

Speaking beside Mayor Eric Adams at a news conference, she said that law enforcement in Nevada had told the police department that Mr. Tamura had “a documented mental health history” but did not elaborate. She said he appeared to have acted alone.

“We are still unraveling what took place,” the mayor said.

The rampage took place at 345 Park Avenue, a skyscraper blocks from Rockefeller Center and St. Patrick’s Cathedral. The workplace mass shooting was particularly shocking in the center of New York City, where such attacks are virtually unheard of.

Ms. Tisch said that Mr. Tamura’s car was recorded passing through Colorado on Saturday, then Nebraska and Iowa on Sunday, before he arrived in New York City on Monday afternoon. He drove to Park Avenue, where he double parked his BMW just outside the skyscraper. He then marched inside with an AR-15-style rifle and began to spray the lobby with gunfire, peeking behind a security desk to shoot one of his victims and firing on another who tried to take cover behind a pillar.

He then walked to the elevator bank, waited for an elevator to take him upstairs, letting a woman exit the car and walk safely past him before he got on and rode it to the 33rd floor.

Once there, he went to the office of Rudin Management, a real estate firm, where he shot someone and then fatally shot himself in the chest.

The police department said Mr. Tamura was from Las Vegas, and public records indicate that he spent at least part of his youth in California. Mason Thomas said he had played high school football with Mr. Tamura in Granada Hills, Calif., but lost contact with him years ago.

Nevertheless, he said it was “mind-blowing” to see an old high school teammate, whom he remembered as funny, popular and agreeable, in the news for a fatal shooting spree.

“There was nothing from the little I knew about him that would have indicated anything like this,” said Mr. Thomas. “At practice, there were never any issues I can remember. He never had issues with anybody.”

Records show that a person with the same name as Mr. Tamura held a work card issued by the state board that regulates security guards, private investigators and similar professionals in Nevada from December 2019 to December 2024. The work card did not authorize him to carry a firearm.

It was not clear why Mr. Tamura was in New York City or what had motivated him to open fire inside the building, which is home to the offices of several high-powered companies, including the investment giant Blackstone, the National Football League, and the accounting and financial advisory firm KPMG.

The shooting on Monday was the second high profile shooting in Midtown Manhattan in less than a year. Last December, Luigi Mangione, traveled to New York City from out of town and staged a targeted attack on a health care executive, Brian Thompson, in Midtown.

Kirsten Noyes contributed research. Chelsia Rose Marcius contributed reporting.