It began with a startling, inexplicable explosion of anger over a seat on a D train. And then the passenger who had resisted giving up his seat was bleeding from stab wounds, staggering through the car, collapsing, dying.

The other passengers on the train at 2 a.m. Saturday — among them four photography students on their way home from burgers and beer in a Midtown restaurant — had just become witnesses to a murder. One of the students, Paola Nuñez Solorio, 30, chronicled the aftermath of the stabbing, ending her day as she had begun it, watching the world through her camera.

In the morning she had zoomed in on apartment-building courtyards for a school project. Now she was photographing frightened passengers, a wounded man tottering through the train, a killer standing at the end of the car.

She took 120 photographs in the mayhem that followed the stabbing, some out of focus, some a blur of passengers scrambling to get out of harm’s way, some showing the victim’s blood on the floor, some showing the victim’s body sprawled across the two seats he collapsed on.

New York City and its subways are far safer than they once were, but violence can still erupt, seemingly out of nowhere, and Ms. Nuñez’s photographs captured the harrowing moments in all their unfiltered, unvarnished reality.

Ms. Nuñez and her friends had been standing in the first car of the uptown D train after they boarded at 42nd Street and Avenue of the Americas. There were scattered empty seats, not enough for them to sit together, so they stood, talking about nothing in particular.

The train stopped at Rockefeller Center, and moments after it pulled out of the station, they realized something was wrong at the other end of their car.

“Everyone started running toward us,” Ms. Nuñez said. “We thought there was a fight. Then we saw this guy with blood coming out of his mouth, and the killer right behind him, putting this thing away. I didn’t know what it was.”

It was a knife, other passengers said.

Instinctively, Ms. Nuñez reached for her camera, a digital single-lens reflex model that did not need a flash to function in the fluorescent light of the car.

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.

Once the doors opened and the police stepped in, “We told them, ‘That’s the guy who killed him,’ ” the woman recalled. “They said, ‘Step aside.’ ”

She said Mr. Sanchez tried to blend in with the other passengers as police officers swarmed into the car. “The killer tried to be a part of us,” she said. She remembered hearing him say, “I didn’t do anything.”

Mr. Johnson was pronounced dead at the scene, the police said. Mr. Sanchez was charged with second-degree murder and weapons possession.

National Geographic’s International Photography Contest attracts thousands of entries from photographers of all skill levels around the world every year. While this year’s entry deadline has passed, there is still time to view and vote for your favorites in the Viewer’s Choice competition. National Geographic was kind enough to let me choose a few of their entries from 2009 for display here on The Big Picture. Collected below are 25 images from the three categories of People, Places and Nature. Captions were written by the individual photographers. (25 photos total)