Dakota Shrader thought it was just another routine fire drill at her Texas high school. But when she heard three gunshots that sounded like massive explosions, she knew she had to run.

Shrader and a classmate didn't stop until they'd gotten to a forest near Santa Fe High, she told NBC affiliate KPRC. She had an asthma attack and called her mom. She heard screaming and the blare of sirens.

"I was scared for my life," said Shrader, a 10th-grader, breaking down crying. "Nobody should go through this."

Nine students and one teacher were killed in the shooting at Santa Fe High School, according to several law enforcement sources who cautioned that the number could change.

The murder suspect was identified as Dimitrios Pagourtzis. He was taken into custody shortly after the shooting, officials said. He is charged with capital murder and aggravated assault of a peace officer and made a brief court appearance Friday afternoon.

Witnesses and survivors recounted the sheer terror of the shooting and its frenzied aftermath. Teachers told students to run as fast as they could.

The scariest thing is, hearing a teacher who knows your name personally, call you by your name and tell you to run — Galaneaa (@Mr_thing456) May 18, 2018

Bailee Sobnosky, a sophomore, said she was 200 feet away from the shooter when she was told someone had a gun.

"I started telling all my friends he has a gun, someone has a gun in the school, but they didn't believe me," Sobnosky told MSNBC. "After the second round of shots, everybody scattered, we finally all believed it and ran for the street," she said.

Damon Kaluza, a 12th-grader, said the whole thing "did not feel real."

"People were running, screaming, people were crying," Kaluza said. "[They] feared for their lives, just like any regular human being would have done."

Rome Shubert, a sophomore and pitcher on the school’s baseball team, was shot in the back of the head, with the bullet exiting underneath an ear. He said he’s lucky to be alive.

"The doctor said that if it would have been any up, any down, any left, any right — that I would have been paralyzed or killed,” Shubert said.

I’m so greatful and blessed that god spared me life today. Today I was shot in the back of the head but i am completely okay and stable. — Romeshubert️ (@rome_shubert17) May 18, 2018

The teenager said it was only after he ran out of the school that he realized he had been shot.

"I looked down at my shirt and I was dripping blood everywhere, I was running fine, I was talking, I had no idea that it went in and out," he said. "I immediately took my shirt off and put it against my neck and put pressure on it."

The Class of 2020 student, who has already committed to the University of Houston, said he never expected his school to be the scene of a mass shooting.

"It's kind of surreal. You don't wake up every day preparing for that kind of thing," he said. "You just don’t think it’s going to happen."