A group from the finance firm Blackstone gathered for a mixer off the lobby of 345 Park Avenue on Monday evening. Across the big, airy space a Blackstone senior executive, Wesley LePatner, 43, was passing through after a day of meetings upstairs. She was a mentor to young women who oversaw a real estate team that had injected tens of billions of dollars into their portfolio.

A busy Monday, nearing its end.

There was the lobby’s security guard — friendly and popular. He stepped outside every day to buy a lottery ticket from the news stand on Lexington Avenue. Today’s my day, he would joke with the young vendor. I’ll win big and solve all my problems.

Darin Laing, 37, in finance, passed him by as he left with a colleague to grab a quick dinner across the street.

None of them noticed a dark BMW pull up on Park Avenue and double park. The driver stepped out. It was a hot day, the beginning of a heat wave that gripped the city. So the lobby’s big blinds were lowered against the sun, masking his approach to the building.

Just before 6:30 p.m., the driver, a slim young man wearing sunglasses, entered the lobby with an assault rifle in his right hand.

Much would be learned about that man in the hours and days to follow — and about the four others who would ultimately lose their lives. But at that moment and for a long stretch that followed, he was an anonymous, terrifying, unfolding threat. One that New Yorkers have seen play out all over America, and now had come to their door.

Interviews with building employees and law enforcement officials tell the story of those harrowing minutes.

A police officer working security for the building, Didarul Islam, 36, didn’t immediately notice the gunman. But the gunman locked in on him immediately.

He opened fire, and the mundane workday hum of the big, bright lobby exploded in bullets and screams, blood and shattering glass.

Chaos in the Lobby

Officer Islam — a husband and father of two boys with a third child due in weeks — went down. His commanding officer had praised him just days earlier for his string of good arrests. Here in this lobby, where he picked up shifts for extra money, he may not have seen the shooter.

An employee from the N.F.L. offices upstairs was shot and went down, wounded. Ms. LePatner, the Blackstone executive, moved toward a pillar for cover, but the shooter saw and shot her dead.

The security guard, Aland Etienne, 46, spun around and turned toward another security desk. He was trained to lock down the building’s elevators in a crisis. The man shot him and he fell. He tried to crawl toward a control panel but collapsed before he got there.

The deafening gunshots rang outside the building. Inside his sweltering news stand, Md Jubel Ahmed, 30, froze, then saw 15 or 20 people dash out of the building’s Lexington Avenue door. He was scared and ran, too, without pausing to lock up. Let anybody crazy enough to stick around take his gum and sodas.

Mr. Laing, who had just stepped out for dinner, looked back when he heard the clamor — shots, shots, shots, shots. More than 20, it sounded like. Men and women ran out of the building, yelling “Active shooter! Active shooter!” He ran two blocks and ducked into a deli, his clothes soaked in sweat.

At 6:28 p.m., the first 911 calls arrived — there would be 113 in all. Active shooter at 345 Park Avenue.

News Races Through the Upper Floors

In the lobby, the gunman turned toward a bank of elevators — still operating — that were used to reach upper floors of the 44-story building.

Up there, news spread in different ways. Some workers on the lower floors heard the shots directly. Others saw their phones blow up with calls or texts — the N.F.L. worker in the lobby, shot in the back, called his office to warn everyone before he called 911.

There were a handful of employees working on the 21st floor for the law firm Loeb & Loeb who were unaware of what was happening. Suddenly, a group of complete strangers — Blackstone employees from another floor — burst out of an elevator and charged into their office, shouting about a shooter on the loose. The group barricaded themselves in a partner’s office.

Throughout the building, custodians who had been alerted by their supervisor ran to tell tenants what was happening.

Employees looked down from their windows to see police officers storming toward the building. They arrived four minutes after the first 911 call.

Jonathan Gray, the president of Blackstone, was working on the top floor, as far from the lobby as one could be, when his phone sounded with frantic calls.

Employees reported a shooter. Another colleague called — Ms. LePatner was down and bleeding in the lobby. Mr. Gray had been in a meeting with her that morning, impressed at her total command of the room.

It felt like a sweeping, confused frenzy that you hear about. The fog of war.

The Wrong Floor

In the lobby, the gunman entered an elevator, seemingly blind to where it would take him. It rose to the 33rd floor, and the doors opened to reveal the offices of the Rudin Management Company, which operates the building.

The man was visibly angry — a surveillance camera in the room captured his reaction. This was the wrong floor.

Glass doors blocked his path into the offices. He opened fire.

Down the hall, Sebije Nelovic, a 65-year-old custodian for Rudin for 27 years, was dusting a table when she heard the noise. She thought it was the engineers upstairs — strange sounds were not unusual up there. She went out for a look.

The glass doors to the elevators were riddled with holes and webbed cracks, and they were shattering as bullets tore into the walls near her.

“God help me,” she thought.

She raised her hands and, in an appeal to whatever moral code the shooter might hold, said that she was just the cleaning lady. She could see he did not care. She turned and ran, holding her garbage can. The shooting didn’t stop.

Julia Hyman, 27, was a Rudin associate and one of the handful of employees still at work. She often stayed late, and greeted Ms. Nelovic every evening.

When the shooting began, she was in a bathroom. She stepped out into the hall. The gunman shot her in the back. She crawled toward a desk, reaching for a phone, and collapsed.

Pleas for Outside Help

At Blackstone offices several floors below, a group of employees scoured their floor looking for somewhere to hide, finding a wellness room that no one knew was there. Some 20 people herded inside. There was one chair.

Trapped in rooms like this, frightened tech-savvy employees reached out — to the world — for information.

“Hi I’m barricaded in a bathroom,” someone posted on the Reddit channel r/AskNYC. “Please let me know what you hear we haven’t heard anything.”

“My wife is barricaded in the bathroom with about 20 other women,” someone else wrote.

Amid the chaos, it was easy to miss a new detail. The shooting had stopped.

‘Shooter Apparently Dead’

At 7:46 p.m., a little over an hour after the first 911 call, an N.F.L. executive texted a colleague: “Shooter apparently dead.” The same news quickly reached the Blackstone offices above. Everyone was told to remain sheltered.

The Reddit channel posted dire warnings without evidence. “There is potentially another shooter around,” one user wrote, “so stay where you are and be as quiet as you can.”

The police cleared the building floor by floor, starting at the top and working down. Room by room, knocking first and breaking down doors if no one opened up.

They found the gunman on 33, dead from a self-inflicted gunshot wound to the chest.

Officers sought other threats, finding none. It was after 11 p.m. when the last employees huddled in the building were finally allowed to leave. They were instructed to exit with their hands over their heads, so the police knew they were safe.

Details emerged throughout the night. The shooter, Shane Devon Tamura, was from Las Vegas. A note in his wallet said he believed he had chronic brain damage from playing football in high school, and he blamed the N.F.L. This apparent target was located some 25 floors below the Rudin floor where he went.

The people who had counted 20 or more shots were correct — he fired 23 in the lobby and 24 on the 33rd floor, the final one into his chest.

The details from the event, in their unplanned randomness — the lowered shades in the lobby, the big gun no one noticed outside the building, the wrong elevator bank — reverberated all week, in calls and texts and pings, as the employees from 345 Park Avenue logged on from home.

Stefanos Chen, Jonah E. Bromwich, Taylor Robinson, Ken Belson, Dan Barry and Matthew Haag contributed reporting.