A gunman armed with an assault-style rifle walked into a Midtown Manhattan skyscraper on Monday evening and began firing, killing a New York City police officer, fatally shooting three other people and critically wounding a fifth before killing himself, officials said.

Three of the four people killed were shot in the building’s lobby as the gunman sprayed the area with bullets, Jessica Tisch, the police commissioner, said at a news conference. The fourth was killed on an upper floor after the gunman took the elevator there, Ms. Tisch said. He then shot himself in the chest, she said.

Tenants of the building where the shooting occurred, 345 Park Avenue, went into lockdown mode as fear and anxiety coursed through their offices at what should have been close to the end of another sweltering summer day.

Darin Laing, who works in the building, said he had walked out with a colleague at about 6:30 p.m. to grab dinner nearby before heading back to work. Within seconds, he heard about 20 gunshots in rapid succession.

“My co-worker was like, ‘Did you hear that, did you hear that?’” he said, adding that he had spun around to see what looked like smoke coming from inside the building.

Then he heard screams and saw men and women in business attire bursting through the doors, swarming the block and sprinting in every direction, crying out as they ran.

The slain officer, who was in uniform and working at the building in a security role, was identified as Didarul Islam, 36. An immigrant from Bangladesh, he had been with the department for three and a half years, Mayor Eric Adams said at the news conference. Officer Islam and his wife had two children and were expecting their third, Ms. Tisch said.

“He made the ultimate sacrifice, shot in cold blood wearing a uniform that stood for the promise he made to this city,” she said. The names of the other victims were not immediately released because their families had not yet been notified, Ms. Tisch said.

The shooting occurred less than eight months and only a 15-minute walk from the site of another high-profile Midtown attack: the murder of a health insurance executive who, the authorities say, was killed by a man angry over what he felt were the health care system’s inequities.

Ms. Tisch identified the man responsible for the rampage on Monday as Shane Devon Tamura, 27, of Las Vegas. His motive and reason for targeting the Park Avenue building were being investigated, she said.

“We are still unraveling what took place,” Mr. Adams said.

The building, between East 51st and 52nd Streets, is owned by the Rudin Management real estate firm, one of New York City’s largest property owners. It was in the company’s 33rd-floor offices where Mr. Tamura killed his final victim and himself, Ms. Tisch said.

Other tenants of the building include the N.F.L., the investment giant Blackstone Group and the accounting and financial advisory firm KPMG. On Monday, their staffs and those of other tenants were trapped in their offices for nearly two terror-filled hours.

Ms. Tisch, saying the information was preliminary and subject to change, gave the following account of the deadly events:

Mr. Tamura left Nevada in a black BMW registered in his name and drove through Colorado on Saturday. He made his way across Nebraska and Iowa on Sunday and was in New Jersey as of around 4:30 p.m. Monday. He drove into Manhattan sometime after that.

The city’s 911 call center began receiving calls of a shooting in progress at about 6:30 p.m. Shortly before that, surveillance video shows, Mr. Tamura had double-parked his car on Park Avenue outside the building he would soon turn into a shooting gallery.

He got of the car and walked toward the building’s entrance with an AR-15-style rifle in his right hand. Once inside the lobby, he turned to the right and immediately opened fire on Officer Islam.

Next, Mr. Tamura shot a woman who was trying to hide behind a pillar, and then continued on through the lobby “spraying it with gunfire,” Ms. Tisch said. He shot a security guard who tried to take cover behind a security desk.

The guard and the woman both died. Mr. Tamura shot another man, who, while hospitalized in critical condition, later identified him as the assailant.

At that point, Mr. Tamura called the elevator. When it arrived, a woman got off, and he allowed her to walk past him unharmed. He then rode to the Rudin firm’s offices on the 33rd floor, where he shot his final victim and then walked down a hallway and took his own life.

A search of his car yielded a rifle case, a loaded revolver, ammunition, a backpack and medication that had been prescribed to him. Ms. Tisch said he had a gun license in Nevada and that law enforcement officials there said he had a “documented mental health history.”

The section of Manhattan where the shooting occurred, Midtown East, is a vibrant business district that is also home some of the city’s best-known landmarks, including St. Patrick’s Cathedral and St. Bartholomew’s Church. Grand Central Terminal is a short walk away.

With 345 Park Avenue locked down, hundreds of officers, joined by state and federal law enforcement agents, flooded the area, scrutinizing the insides and trunks of cars parked nearby as police drones and helicopters hovered overhead.

By about 7 p.m., people had begun to march out of the building in small groups with their hands up, according to a witness, Ben Ryder Howe, a freelance journalist.

A short time later, a larger group left the building, with some people running away as fast as they could and others walking normally, seemingly oblivious to the violent chaos unfolding in their midst. Many had their hands up, presumably to show that they were not armed.

A Blackstone employee who spoke on the condition of anonymity because of a company policy against talking to reporters said he and others who work on the building’s 31st floor had heard a loud bang from above at around the time of the shooting.

Some people shrugged off the noise, the Blackstone employee said, while others rushed for the exits. They soon received an email notifying them of a gunman in the building. A follow-up email urged them to evacuate, he said, though by then police officers were ushering people out a floor at a time by elevator.

Jorge Jimenez, a livery cabdriver from Queens, was picking up a client at a restaurant across the street when he saw office workers running with their hands up. One woman was running while holding her high-heel shoes. He said a police dog had searched his car.

“I thought it was like something like terrorism from the commotion on people’s faces,” Mr. Jimenez said.

Hours after the shooting, streets were closed from East 55th Street to East 48th Street as law enforcement officers continued to maintain a heavy presence and look for clues that might help explain what was behind Mr. Tamura’s lethal spree.

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, a neighborhood in Los Angeles, but that they had fallen out of touch years ago.

Mr. Thomas said it was “mind-blowing” to see a former high school teammate in the news under these circumstances.

“There was nothing from the little I knew about him that would have indicated anything like this,” Mr. Thomas said.

In the Parkchester section of the Bronx, about 11 miles from where Officer Islam was killed, fellow officers shuffled in and out of the two-story home he had bought for his family and aging parents. A child wailed inside. The imam of a local mosque consoled the family.

Marjanul Karim, a close family friend, said Officer Islam had been a mentor to young men in a Bangladeshi community that numbers more than 100,000 in the city.

When Officer Islam joined the Police Department, Mr. Karim said his own mother had asked him why he would pursue such a dangerous job.

“He told her he wanted to leave behind a legacy for his family,” Mr. Karim said, “something they could be proud of.”

Reporting was contributed by Wesley Parnell, Bianca Pallaro, Maia Coleman, Michael Wilson, Liam Stack, Joseph Goldstein, Amy Julia Harris, Francesca Regalado, Chevaz Clarke, Santul Nerkar and Matthew Haag. Kirsten Noyes contributed research.