She and her friends had not noticed the suspect, identified by the police as Geraldo Sanchez, 37, an exterminator from the Bronx, when he boarded at Rockefeller Center. Nor had they realized that he had gone on a rampage at the other end of the car, spilling food and demanding that a passenger, Dwight Johnson, 36, vacate a seat beside one of the doors.

Mr. Johnson said there were other seats.

Mr. Sanchez said he wanted that seat. Soon he pulled out a knife and slashed Mr. Johnson’s hands and neck, witnesses said. Ms. Nuñez said Mr. Johnson staggered through the car, bleeding, with Mr. Sanchez right behind him.

“We didn’t know what to do,” Ms. Nuñez said, and she and her friends realized they might be in danger: “We were stuck with the killer.”

The fear behind that thought deepened when someone pulled the emergency brake cord, freezing the train between Rockefeller Center and the Seventh Avenue station. The passengers were trapped in the car with the suspect and the dying victim longer than if the train had gone on to the station and had been met by the police.

As the train sat in the tunnel, the terrified passengers huddled together. Mr. Sanchez walked by and began trying to open the locked door leading to the next car. “We didn’t know what to do,” Ms. Nuñez said. “A guy standing with us said, ‘Don’t move.’ ”

Another passenger, a woman, fainted. The man with her tried to revive her, Ms. Nuñez said.

After several long minutes, the train started moving again. It pulled into the next station, at Seventh Avenue, but the doors remained closed.

One of Ms. Nuñez’s friends said it appeared that the two officers on the platform were waiting for backups to arrive. That left the passengers in the car feeling vulnerable anew. “People were looking at us like we were in the zoo,” said the friend, who asked not to be identified because she had not told her father that she had been at the scene.