Michigan State University student Dominik Molotky was in class on Monday evening when he heard a gunshot in the hallway, just outside the door.
"I was sitting next to the nearest door and thank god that my fight-or-flight response kicked in because, right when that first gunshot went off, I booked it to the far corner of the class," Molotky, a senior, told ABC News in an interview Tuesday on "Good Morning America."
A couple seconds later, the gunman entered the classroom and fired "three to four more rounds," Molotky said.
"I was ducking and covering," he recalled. "I think one of the students in my class got hit."
When the gunfire stopped for "30 seconds to a minute," Molotky said, he and his classmates started breaking open a window so they could escape.
"There was glass everywhere," he added. "We broke open the window and climbed out of there, and I booked it back to my apartment."
Molotky and other students recounted their horrifying experiences as the mass shooting unfolded at multiple locations on MSU's main campus in East Lansing, Michigan. At least three people were killed and five others were wounded, all of whom where students, according to police. The suspect -- identified as a 43-year-old man with no known ties to the university -- was later found dead from an apparently self-inflicted gunshot wound, police said.
MSU graduate student Graham Diedrich said he was working at the writing center in the university's library on campus when he heard police sirens outside. He noticed other students looking at their cellphones with concern, so he checked his email and found a message from the university offering advice on what to do during a mass shooting: "Run, hide, fight."
"That's when we kind of knew what was going on," Diedrich told ABC News in an interview late Monday.
Diedrich said he never heard any gunshots, but he and other students decided to barricade themselves in a study room in the back of the library for safety. They stacked chairs, tables and a shelving unit to block the glass door and windows.
"We wanted to protect ourselves and make sure that there was no glass, no way that you could see into the room," he added. "We dimmed the lights."
Diedrich said they were told by the university and police to shelter in place. They were still barricaded in the library when Diedrich spoke to ABC News.
"I didn't think that it would happen to me. But living in America, you know, you always have to live under the assumption that this will happen to you. It happens to people every day in this country, and we have to consider why," he said. "It's not unique, but it's a lot different when it happens to you."
MSU students Luke Restrepo and Riley Dorfman said they were in their apartment when they noticed a scene of "absolute chaos" outside their window -- police cars speeding by and people running for their lives.
"I called my mom and she was in shock, she was terrified," Restrepo told ABC News in an interview late Monay. "You never want your kid to call you and say: 'Oh hey, by the way, the school is under attack, it's getting shot up a block away from me and I cant guarantee my safety.'"
Dorfman added: "The fact that this has been going on for three hours now and I have to constantly call my mom every 20 minutes to hear her crying, hoping I'm OK, it's not OK. It shouldn't happen."