GRAND RAPIDS, Mich. (WOOD) — Michigan State University assistant professor Marco Diaz-Munoz was teaching a class on Cuban Cultural Identity Monday when the door of his classroom opened and a stranger opened fire. He spoke with NBC News about the shooting that killed two of his students, hoping it will bring about change.

Arielle Anderson, Brian Fraser and Alexandria Verner were killed in the shooting. Two of them were in Diaz-Munoz’s class.

Arielle Anderson, Brian Fraser and Alexandria Verner.

The class of about 40 students was in Berkey Hall, where the first shots were fired around 8:15 p.m.

“We were talking about things students find interesting. That day had to do with pirating in the Caribbean … when all of the sudden we heard these explosions outside,” Diaz-Munoz said in an interview with NBC News.

He said he thought the noise was a transformer or something electrical sparking. It never occurred to him it might be gunshots.

“Within seconds, because there were three loud noises, all of a sudden, there’s this individual — I don’t even know how to describe it to you because it didn’t even seem human,” he said, explaining that the gunman was covered from head to toe.

“(I just saw) sparks, like shots and shots and shots,” he said.

Someone yelled, “A shooter” and students started ducking and running.

“This guy kept on shooting and shooting,” he explained to NBC. “It seemed like an eternity.”

“All this time while this is happening, the only thing that runs through I imagine everyone’s mind but definitely through mine is ‘This is not happening. This is not happening. This cannot be happening.’ There’s this minutes, 30 seconds of disbelief while you’re hearing the shots (that) you think, ‘This is not real. This is a joke,'” he told NBC News.

Diaz-Munoz estimated the gunman got off at least 15 shots before leaving the room.

All Diaz-Munoz could think to do was to block the second door to the classroom that was near him. He couldn’t see a lock, so he used his own body to hold it closed. For the next 10 to 12 minutes, the classroom waited.

“No one knew where he was,” Diaz-Munoz said.

Diaz-Munoz told his students to kick the windows out and run. Between 10 and 15 got out of the room that way. Others, he said, started first aid for the injured.

When the police arrived, Diaz-Munoz said he first thought they were the shooter coming back. Once he realized it was police, he left the door and went to see who he could help. Paramedics started treating the injured, but Anderson and Verner, who had been sitting near the back of the room, were already gone.

“I learned later that they didn’t make it,” Diaz-Munoz told NBC. “But I’m haunted by that. I’m haunted because I didn’t know what to do.”

The five injured students were taken to a local hospital. Four remained in critical condition Friday. One was in stable condition.

Everyone who hadn’t escaped was escorted to the Eli and Edythe Broad Art Museum. They were there for about three hours.

The shooter ultimately took his own life when confronted by police about 4 miles from campus.

Diaz-Munoz said he hasn’t been able to escape the “very vivid and clear” images in his mind. He struggles to sleep. Monday marks one week since the shooting and the day classes will begin again at MSU. Diaz-Munzo said he’s eager to get back with his students.

“Those 12 minutes of horror brought us together as a community. Those kids now feel to me like family,” he told NBC News.

“Part of me doesn’t want to teach that class, definitely not (in) that building ever again, but there’s a part of me that knows those students, the same way that I need to see them and see that they are alive and are well, I’m sure that there’s a part of them that also wants to feel like their professor is teaching them because that brings them a kind of normalcy to their life,” he continued.

Diaz-Munzo told NBC News he’s hoping there will be action.

“I think if the Senate and lawmakers of this country saw what I saw, they would be shamed into action or their humanity would be touched into action,” he said. “…I believe that the biggest changes in human history have happened when people let their humanity lead them into action, when they have been touched, when they have been more let their human emotions and passions (lead) their decisions.”