On Monday evening, Sebije Nelovic was dusting a table on the 33rd floor of the Park Avenue office building she had been cleaning for close to three decades when she heard startling noises.

She stood, looked toward a glass door separating her from the elevator bank and saw a man holding what looked to her like a machine gun.

He pointed it straight at her.

During the 27 years she has been cleaning the building at 345 Park Avenue in Midtown Manhattan, Ms. Nelovic, like so many other Americans, has watched mass shootings play out in malls and schools across the country, feeling so sorry for the victims and helpless to do anything about it.

On Monday night, Ms. Nelovic, 65, experienced that terror firsthand.

The day began as her days normally do, she said in an interview on Thursday. She reported for work around 4:30 p.m., preparing her cart and the other materials she needed to clean the 33rd floor. Then she began to pick up the garbage on the 51st Street side of the building.

There were five office workers still there, including Julia Hyman, 27, an associate at Rudin Management. Ms. Hyman was friendly with the woman who cleaned her office nightly, and they exchanged the usual greetings.

Ms. Nelovic finished up that side of the building, threw the garbage she had collected in a freight elevator and moved to the 52nd Street side. It was about 6:10 p.m.

She took a short break and then moved toward the Lexington Avenue side of the building. She had begun to dust a table when she heard noises that initially sounded like firecrackers.

She thought the sounds might be coming from a group of engineers who worked on the 34th floor and occasionally made strange noises. But she soon realized they weren’t coming from the engineers, nor were they firecrackers.

She stood and saw, through the glass door, the gunman. When he saw her, he pointed the rifle at her. He looked young, she said, but that was all she really remembered, other than the gun, and the bullets.

The man, 27-year-old Shane Devon Tamura, shot through the glass, and the reception door began to crumble.

“God help me,” she thought. “God help me.”

Ms. Nelovic — one of 111 members of 32BJ SEIU, the office workers’ union, employed at the building — raised her hands and told him that she was the cleaning lady, but she soon realized he did not care. The bullets were coming through the door and the door was coming down. Ms. Nelovic turned and ran back down the 52nd Street side of the building.

She did not look back, nor did she see Mr. Tamura again, though she continued to hear him shooting.

She felt he was going to keep shooting forever.

She passed the boardroom, and the conference room and found a closet, where she locked herself inside. She began to pray, asking that God move the man away from her.

Her supervisor called and asked her where she was, then warned her to stay put on the 33rd floor because there was a gunman in the lobby.

“The guy is on my floor!” she said, shocking the supervisor.

She continued to hear the gunshots. They were constant, but she just kept praying. A bullet hit the door of her closet.

When her supervisor called again, she demanded no more calls; she was terrified that Mr. Tamura would hear them. She turned down the volume of her phone.

The gunshots were still reverberating all over the floor and at one point she heard bullets pass close by. She heard someone scream, and thought of the office workers she had seen earlier, including Ms. Hyman.

Ms. Hyman had always worked late, sometimes until 8:30 p.m. But every evening, no matter how late she had worked, she would find Ms. Nelovic and say good night.

Mr. Tamura killed her.

Ms. Hyman was among four victims, including a Police Department officer. The gunman had driven from Nevada, and may have been planning to attack the N.F.L. offices in the building. He seems to have gone to the 33rd floor, far above the league’s offices, by accident. He ended the massacre by shooting himself in the chest.

Ms. Nelovic was still in the closet when the gunshots stopped, but she was too scared to come out. Her supervisor told her that the police would open the door, and soon they did.

Officers brought her to the reception area, and then to a car, but she felt terrified just being at the building and told them that she didn’t want to stay there. Eventually, she left.

In the interview on Thursday, she teared up thinking of Ms. Hyman, whose funeral had been held the day before.

“She was so nice,” Ms. Nelovic said. “I feel so sorry for her. I feel so sorry for everyone.”

She will not go back to 345 Park Avenue.

“Now, no more,” she said. “I’m scared to go there. God help me.”

Dana Rubinstein contributed reporting. Kitty Bennett contributed research.