The 15-year-old gunman eventually shot himself as police stormed the room at Marinette High School hours later Monday evening, and he was in a grave condition early Tuesday.
The teenager allowed five of his hostages out after about six and a half hours, and finally all 23 and their social studies teacher Valerie Burd emerged unharmed. Student hostage Zach Campbell said the gunman seemed depressed, but he didn't think he meant his classmates any harm.
"He didn't really want to hurt anybody," Campbell told CBS' "The Early Show". "He just … he was very depressed. You could kind of tell. So he didn't want to shoot any of us."
Campbell told The Associated Press that six of the gunman's close friends were in that class.
Authorities also said they did not know what might have motivated the boy who made no demands or requests during the standoff.
"As far as what caused this, it seems to be a mystery," Marinette Police Chief Jeff Skorik said early Tuesday. "We have not been able to identify anything that precipitated this incident."
Skorik said the suspect fired three shots immediately before police entered the room, but he had also fired at least two or three shots before that. He shot into a wall, a desk and equipment in the room, but he was not aiming at any students, Skorik said. The shooter was carrying a 9 mm semi-automatic and a .22 caliber semi-automatic, and he had additional ammunition in his pocket and a duffel bag with more bullets was found at the scene, the chief said. A knife was also found in the room, he said.
A bomb-sniffing dog was brought in to check the building for explosives and none were found, the chief said. He said it was not clear where the boy got the weapons or how he sneaked them into school.
Speaking on ABC's "Good Morning America," student Austin Biehl said the teacher asked the gunman why he was holding them hostage.
"He just said 'no,' that he didn't want anything, didn't want any help," said Biehl, who was so scared that his legs were shaking.
The gunman was taken to a nearby hospital. Authorities have declined to release his name. Skorik said it was his understanding that the boy was in grave condition Tuesday morning.
"The information that I've received is from officers on scene," the chief said. "There was quite a bit of medical treatment going on."
The shooter entered the classroom, where he was a student, at around 1:30 p.m., Skorik said. It wasn't until the end of the school day, more than two hours later, that the principal learned that neither the teacher nor any of the students from the class had been seen, Skorik said. He went to investigate and was threatened by the shooter to "get out of here," Skorik said.
Campbell said the class was watching a movie when the gunman shot the projector, then fired a second round. He had two handguns and refused to let anyone leave, Campbell said. He demanded everyone dump their cell phones in the center of the room. When the gunman's own cell phone rang, the boy snapped it in half, Campbell said.
"We just kind of stared in disbelief. Nobody really knew what to do, you know. Pretty scary," Campbell said, adding that he was "just kind of hoping to get out alive."
He wasn't interested in talking with the teacher and told her to be quiet, Campbell said. But the gunman chatted with his fellow students, who tried to keep him talking about how he hunted and about fishing. Students even got the gunman to laugh, Campbell said.
The gunman refused to communicate with officials during the standoff, Skorik said, but allowed the teacher, Burd, to speak with them by phone.
"The teacher was nothing short of heroic," Skorik said. "I think she kept a very cool head. She was able to keep the suspect as calm as possible. I heard that she took the responsibility of trying to assure the other students they were going to be OK. We really give that teacher a lot of credit for being able to keep a cool head under a stressful situation."
Choral teacher Bonita Weydt said she was talking with a teacher in another classroom at the end of the day when Principal Corry Lambie came in.
"I said, `Corry, what's going on?' and he said, `Get out of the building,"' Weydt said.
Firefighters kept people away from the school. Anxious parents met throughout the evening with officials at the county courthouse.
After about seven hours, the boy let Campbell and four other students out to use the bathroom. Police outside the classroom whisked them to safety.
About 20 minutes later, Skorik said, officers heard three shots and broke down the door. The gunman, who was standing at the front of the classroom, shot himself as officers approached, the chief said.
Students were taken by bus to the courthouse, where they were reunited with their parents.
Keith Schroeder, a former Marinette middle school teacher, said he had the gunman as a student and also knows the boy's teacher well. He said the teen's family is extremely involved in all their boys' lives.
"He's a fine young man, and I'm totally taken aback," Schroeder told The Associated Press. "Surprised, flabbergasted to say the least because this is a great family. It doesn't fit any of the things or the molds that you read about people. I couldn't say enough good things about the family."
Skorik said the district attorney was reviewing the case and would decide whether to file any charges.
Marinette, a city of about 12,000 people, lies about 50 miles north of Green Bay on the border with Michigan's Upper Peninsula. About 800 students attend the high school, according to its website.
Marinette Schools Superintendent Tim Baneck noted the community went through an emergency response training exercise last year.
"So the local law enforcement officials as well as the educators were all involved in a mock shooter situation, so it is actually very fresh in our minds in terms of the training we just went through," he said.
City Councilman Bradley Behrendt said the district spent "a whole bundle of money" on classroom doors to make them more secure, but the school doesn't have metal detectors.
Authorities said the school would be closed Tuesday. District officials said they planned to offer counseling for students.