Ed Andrieski / Associated Press

CENTENNIAL, Colo. — Armed with a shotgun, a student entered Arapahoe High School and opened fire, hitting at least one other student before turning the weapon on himself, officials said Friday.

One student was injured when he confronted the gunman, Arapahoe County Sheriff Grayson Robinson said at a televised news conference. That student was reported in critical condition in surgery at Littleton Adventist Hospital.

Authorities rushed to secure the building and found a second injured student, but it was unclear if that was the result of the shooting, Robinson said.

PHOTOS: Shooting at Arapahoe High School in Colorado

The latest attack comes as the nation prepares to commemorate the first anniversary of the massacre at Sandy Hook Elementary School in Newtown, Conn., where 20 children were killed by a lone gunman, Adam Lanza. Six adult educators also died in that shooting before Lanza committed suicide.

Centennial is roughly 15 miles south of downtown Denver and less than 10 miles east of Columbine High School in Littleton, where two teenage shooters killed 12 classmates and a teacher, then killed themselves in 1999.

The incident began about 12:30 p.m. at the school, and authorities, mindful of recent mass shootings, including one at a movie theater in nearby Aurora, Colo., responded in force. SWAT teams swarmed the school and bomb squads responded.

Blaise Potvien, 14, a freshman, said he was in his fifth period class -- U.S. history -- when he heard three to four gunshots in the hallway.

“It was the loudest thing I ever heard,” Blaise said as he gathered with other students at the nearby Shepherd of the Hills Church. Parents rushed there to greet students who left the school in an orderly march, arms raised.

Seconds after the booming sounds, Blaise said, counselors and staff members were running down the hallway ordering everyone to close the classroom doors.

He said they turned out the lights in his classroom and hid in a far corner. Students were crying and screaming.

He said he texted his mother: “I love you mom and dad. Thank you for a wonderful life.”

Teresa Potvien, his mother, said she was Christmas shopping when she got that text. “I almost threw up,” she said.

She said she jumped in her car and tried to make her way to the school but traffic was snarled. Police cars and fire trucks clogged the road while helicopters filled the sky.

While many students were evacuated from the school, Blaise and other students who were close to where the shootings had taken place stayed hidden until a police tactical team came in and told them it was safe to leave.

John Spiegel, an 18-year-old senior, said he was in his psychology class on the north side of the high school building when he heard a popping sound.

The sound brought the class to a halt, he said, and everyone began looking around, trying to identify it. Seconds later, there were three more rapid shots.

“It was clear as day. It sounded right outside the door,” Spiegel said.

At the same time, he said, he heard what sounded like a student screaming: “We need help!” Students in his classroom ran to the front of the room, turned off the light, and huddled together.

“We were just clumped together on the floor. It felt unreal,” Spiegel said.