Then, just before 11 a.m. Thursday, chaos erupted when Armando Torres, a 35-year- old Queens resident, burst in with a carving knife with an 8-inch blade, the police said.

The community affairs officer started to say, “Can I help you?” according to the police. An officer stationed at the switchboard also saw Mr. Torres, but saw the weapon as well, and shouted, “Knife! Knife!” the police said.

In the space of a few minutes, a man would be stabbed, three officers would fire their guns, and Mr. Torres would be shot and hospitalized.

Police officials were still trying to determine why Mr. Torres would stab someone who appeared to be a stranger, and why he would enter a police station brandishing a knife. But through the accounts of witnesses, the police were able to detail how the violence began, and how it ended.

On the steps of the station house, at 37-05 Union Street, Mr. Torres encountered Armando Ferreira, 48, who had come to pick up an accident report. Mr. Torres nicked Mr. Ferreira in the back from behind, and then slashed him on the arm when Mr. Ferreira tried to defend himself, the police said.

Then, Mr. Torres ran into the station, the police said. Knife in hand, he ran past the community affairs officer, past a desk and toward some stairs at the back leading to the detective squad on the second floor, the police said.

An officer and the detective near the desk gave chase. So did another detective, who had been taking a domestic complaint from Stephen Squerciati, 37, a construction worker, and his fiancée, Marie Woychowski, 35.

Mr. Squerciati said Mr. Torres tripped and fell as he ran through the room. “This guy was waving a knife up the stairs,” he said.

After Mr. Torres got up and started climbing the stairs, the officers ordered him to come down and drop the knife, the police said. Mr. Squerciati said that he heard the command 7 to 10 times.

Mr. Torres came down the stairs but did not drop the knife, said the Police Department’s chief spokesman, Paul J. Browne. At the bottom of the stairs, he “comes at them with the knife,” Mr. Browne said.

By now, the two detectives and the officer had drawn their weapons. They fired five shots, striking Mr. Torres several times in the left arm and left side of the chest, Mr. Browne said. Mr. Torres took a few steps into the room, stumbled and fell to the floor, breaking off the knife blade from the handle, Mr. Browne added.

The two detectives and the officer had never fired their weapons at a suspect before Thursday, Mr. Browne said. The three officers, whose names were not released by police officials, all tested negative for alcohol, Mr. Browne said, which is a routine measure the department takes after any police shooting.

Mr. Torres was taken to New York Hospital Medical Center of Queens and was in stable condition on Thursday, the police said. He faces charges of weapons and felony assault, they said.

“If there is one constant in the N.Y.P.D., it is that your world can change in a heartbeat, even in the station house,” said Mr. Browne, who said the police had not determined Mr. Torres’s motive.

It was not clear whether there was any link between Thursday’s attack and a previous arrest of Mr. Torres, also within the 109th Precinct. In 2007, according to a criminal complaint, Mr. Torres was accused of beating his roommate on the head with a metal bar and then slashing his midsection. He pleaded guilty to a lesser assault charge and later received a conditional discharge.

At a residential building at 147-25 Northern Boulevard, where Mr. Torres lives, neighbors said the ex-wife and children of his brother, Nelson, were staying there with him. Mr. Torres has lived in the apartment for about eight years, friends and people in the building said. Apparently, they said, Nelson, who lived there, too, moved out after having an argument with his ex-wife , but remains close to his brother.

Residents at the building said Mr. Torres worked in construction or landscaping.

A man who identified himself only as Javier, 25, who works in the building, said Mr. Torres was from El Salvador. “He seemed like a normal person,” he said. “He would drink sometimes. I saw him yesterday in the laundry room around 11 a.m.”

Mr. Ferreira, a construction worker, later returned to the station to speak to the police. He said that he had been reading his accident report on the steps of the station when Mr. Torres came up a ramp behind him, nicked him in the back with the knife and slashed him on the left forearm badly enough to require three stitches.

Mr. Ferreira said that he dashed into the station house and that Mr. Torres, who did not say a word to him during the attack, ran past him into the building.

“I heard a lot of yelling from the police and demands from them that he drop the knife,” Mr. Ferreira said. “For me, they did their job to protect people.”