(CNN) The day was almost over and students were puzzled because the fire alarm was going off a second time Wednesday.

Some students headed out as if it were a drill, but their fun was suddenly interrupted by a burst of gunfire.

It soon became clear there was a killer in their school and their lives were in danger.

Here are the words of some of the students who went through the tragedy that took at least 17 lives at Marjory Stoneman Douglas High School in Parkland, Florida:

Teachers reportedly injured, killed

Tyjanai Thomas told HLN's Ashley Banfield she saw a teacher getting shot by the gunman.

"He stopped in front of our door because the door was open and he was trying to get to more students. Our teacher started to like interfere and protect the students so he shot him."

"We heard it (firearm) go off and he fell down," she added.

CNN affiliate WSVN spoke to a student who said her teacher was killed. The girl, identified only as Alex, said she and her classmates were leaving because of a fire alarm.

"People were halfway down the stairwell, it just stopped, the alarm stopped. And we heard gunshots coming from the first floor, the second floor and people were running upstairs. We all got upstairs and into our classroom. As (my teacher) was closing the door he was actually shot and killed right there. The door was left open the whole time so as (the shooter) walked by, the door was open. He could have walked in at any time."

'Go back, go back'

Zaphena Jasmin told CNN she sheltered in a classroom and prayed with a friend in the corner of the room. She said she didn't initially move when the alarm went off and was turned around while trying to leave the building.

"I didn't really budge. ... Then I hear on the announcements to evacuate. I'm halfway down the stairs and an administrator is blocking my way, and saying, 'Go back, go back.' After he said that I heard four or five gunshots and we all started booking it to a classroom that was open. We heard a bunch of pop, pop, pop. So if it wasn't real gunshots it was the drill that they were training us for, but either way I took it seriously."

'I heard shots'

A boy told WPLG he heard shots about 20 minutes before the end of school. Everyone headed outside to their appointed meeting areas.

"At 2:19 p.m. I heard shots (others say the alarm and shots came later). We ran outside. No one knew anything that was happening because the fire alarm already went off that day in second period. So (during the afternoon alarm) everyone was messing around. Once we heard (the shooting) we were running outside. We were going in our zones; we have certain zones that we have to be in. I started hearing like six gunshots like far away, but I could still hear them clear as day. And as soon as that ended, I told everyone we gotta go. We gotta run."

Sounded like firecrackers

A girl who gave her name as Isabella told WPTV the scene was very chaotic because of the blaring fire alarm, especially because it was going off just before the end of the day.

"I heard screaming. I heard about five, six gunshots. We thought they were firecrackers because it sounded like them. We weren't sure what was used. We heard the police yelling. We heard banging on the doors."

Never heard the shots

A girl who had evacuated for the fire alarm said there were several hundred students who never heard the gunshots. They didn't think much of it until police cars showed up at the school, Sarah Chadwick said.

"I never thought something like this would happen, especially in Parkland, Florida. We're known for being one of the safest cities in Florida. My whole class ... evacuated (during the fire alarm) and went to the back of this big field that we have. There were a lot of us there, about 200. Then all of a sudden we see tons of cop cars. We see cops coming toward us, security on golf carts coming and that's when teachers and security guards and cops were telling us to run (to a nearby Walmart). We didn't hear them from that far away but as the gunshots were happening some of my friends were texting (from) classrooms right near the gunshots."