Three Booms. A Masked, Armed Man. How Horror Unfolded in a Michigan State Classroom.

EAST LANSING, Mich. — Prof. Marco Díaz-Muñoz was standing at the front of Room 114 on the first floor of Berkey Hall on the Michigan State University campus on Monday evening, flipping through PowerPoint slides before a roomful of several dozen undergraduates.

Suddenly, he heard a loud boom from outside the long, narrow classroom. Then another. And another.

Probably a transformer, Mr. Díaz-Muñoz remembers thinking, something electrical that has gone wrong with the building. A moment later, he saw a figure at the doorway.

“It looked like a robot, not someone human, covered with a mask and a cap,” he recalled in an interview on Thursday. “It seemed just unreal.”

The man stepped into the room, and Mr. Díaz-Muñoz, 64, glimpsed something silvery or metallic in his hand. The man raised it in the direction of the students and began to fire.

“A shooter!” one of the students yelled out. The gunman did not speak.

What had begun as a typical Monday evening, in a class for undergraduates on Cuban cultural identity, instantly became a scene of death and terror. Mr. Díaz-Muñoz said he watched as the gunman, identified by the police as Anthony McRae, 43, shot at least seven students in the classroom, killing two of them.

“I have never had the experience of hearing gunshots close to me,” he said. “Now I will never forget.”

After the shooting began, Mr. Díaz-Muñoz said, he was too shocked to dive for cover. But his students, well trained in classroom shooting drills, reacted instantly. They began to scramble to escape, trying to hide under desks or run toward another door in the classroom, away from the gunman.

Some of them appeared to be slowed down by the design of the chairs, which were clamped to the floor, making escape difficult.

“Everyone who could, threw themselves to the floor,” said Mr. Díaz-Muñoz, who has taught at Michigan State since 2008. “Some had already stood up and were frozen. He was shooting and shooting and shooting, at least 15 gunshots. I don’t know if he ever stopped to reload.”

After what felt like a minute or two, Mr. Díaz-Muñoz said, the gunman left the room.

Two students were killed, Arielle Diamond Anderson and Alexandria Verner. At least five other students were wounded from the gunfire. One student was gasping for air, apparently suffering from an asthma attack.

Mr. Díaz-Muñoz positioned himself at a door, blocking it with his body so the gunman could not return. He told students to break windows so they could escape. They began to smash them and, with some difficulty, climb through the broken glass to the ground below.

Some students remained in the classroom and tended to the wounded. “There was a student by my blackboard curled up saying, ‘It hurts, it hurts,’” he said. “The students who stayed were adding pressure, helping them, so that they wouldn’t bleed to death.” He said he believed those students survived.

Within roughly 10 minutes, the police arrived, and paramedics quickly began treating the students who were shot.

Mr. Díaz-Muñoz and the students who had escaped the gunfire were eventually escorted out of the classroom and to the art museum next door.

At a news conference at the university on Thursday, Chris Rozman, the interim deputy chief for Michigan State, said Mr. McRae had legally purchased the two guns he brought to the campus on Monday night despite a 2019 arrest on felony charges for carrying an unlicensed Ruger LCP .380 semiautomatic handgun.

With no history of violent offenses and only minor scrapes with the law, mostly vehicle charges, he eventually pleaded guilty to a misdemeanor in Ingham County, which includes much of Lansing, and was sentenced to 18 months of probation, which left open the option of purchasing weapons in the future.

When Mr. McRae was found, Chief Rozman said, he had two handguns, along with additional magazines and ammunition. Mr. McRae had no known connection to the university.

“There is always room for some type of discrepancy or discretion,” Chief Ellery Sosebee of the Lansing Police Department said of the prosecutor’s decision to accept the plea deal. “However, that one will be scrutinized for a long time.”

Speaking to reporters at the Michigan State Capitol on Wednesday, Dana Nessel, the state’s attorney general, said that the sentencing guidelines for concealed weapon charges are minimal without any prior offenses.

“At this point, it’s almost impossible for a person to actually serve time for it,” Ms. Nessel said. “So when I see people saying, ‘Well, he should’ve done five years for carrying a concealed weapon,’ first, that’s such a common offense that we’d have to build new prisons.”

The way the laws are written, Ms. Nessel said, it would be “impractical and improbable” for someone to receive a felony conviction for carrying a concealed weapon without significant prior offenses.

Carol Siemon, the Ingham County prosecutor at the time, did not respond to a request for comment on Thursday afternoon. In an email to Bridge Michigan, a nonprofit news site, Ms. Siemon defended her decision, adding that plea deals like Mr. McRae’s were “standard” practice.

“When something awful like this occurs, it is natural to revisit the past, but oftentimes the decisions would be the same,” Ms. Siemon told the news outlet.

Gov. Gretchen Whitmer and Democrats in the State Legislature have vowed to push for stricter gun laws, including universal background checks and red flag and safe storage laws. Efforts to pass gun control measures in a Republican-controlled Legislature were unsuccessful after the mass shooting at Oxford High School in 2021.

A statewide poll in Michigan released in December and commissioned by the Detroit Regional Chamber found that when asked about legislative priorities this year, Democratic and independent voters favored gun control laws above any other issue. Nearly 90 percent of all Michigan voters support background checks before gun purchases, according to the poll.

Chief Rozman said on Thursday that officials were still trying to determine whether Mr. McRae had a motive for targeting the university. A note written by Mr. McRae appeared to indicate he had felt “slighted” by employees and businesses, Chief Rozman said.

“Did a mental health issue amplify that or was it a component of that? We’re not sure at this point,” Chief Rozman said. “We’re working our best to try to determine that as best as possible.”

University officials said on Thursday that one of the victims had improved to stable condition, and that the four other victims were still in critical condition.

Ms. Whitmer said at a vigil on Wednesday night that she met with two victims earlier in the day. One victim told her that a student took his shirt off and pressed on his chest, an act that most likely saved his life.

Classes remained suspended through Sunday, but university offices were open on Thursday, said Teresa Woodruff, the university’s interim president. Ms. Woodruff said that Berkey Hall would remained closed for the rest of the semester. It was still unclear when or whether the student union would reopen, she said.

University officials have also discussed the possibility of allowing students to take classes virtually for the remainder of the semester, but no official decision has been made, Mr. Woodruff said, adding that the university was offering counseling and other services for students.

“None of us have all the answers, but we do have each other,” Ms. Woodruff said.

University officials were also discussing how Berkey Hall and the student union would be used long term, Ms. Woodruff said.

The editorial board of The State News, the university’s student newspaper, published an opinion column on Thursday calling on the university to allow them more time to grieve and process Monday’s shooting before returning to class.

“Our home will never be the same,” the editorial board wrote. “We can’t physically sit in a classroom on Monday. It’s been less than a week since we lost three fellow Spartans in those classrooms. We aren’t ready.”

Mr. Díaz-Muñoz is grieving, too. He spent Tuesday and Wednesday in a state of shock and grief. “I retreated into myself,” he said.

Now he is calling for action from Congress and lawmakers. “If they saw what I saw, if this had happened to their children, they would move into action,” he said. “I saw the horror of these two kids who didn’t need to die.”

Amanda Holpuch contributed reporting.