(CNN) -- No one knows why he picked this day, this time, these victims.
It was the first day back from fall break at Sparks Middle School. Students milled about, waiting to hear the morning bell.
Within moments, two 12-year-old students were wounded. A beloved teacher and military veteran lay dead. And the young shooter -- armed with his parents' gun -- took his own life, silencing any way of understanding what he was thinking.
Photos from the scene
'I think he took out his bullying'
Before Monday morning, the gunman seemed like the antithesis of a school shooter.
"He was really a nice kid," schoolmate Amaya Newton said. "He would make you smile when you were having bad day."
But for whatever reason, the boy, whom authorities have not identified, took his parents' handgun to school, a federal law enforcement source said.
"I believe it was because I saw him getting bullied a couple of times, and I think he took out his bullying," Amaya said.
Amaya said she thought the two students at the Nevada school were friends of the shooter.
But "it's too early to say whether he was targeting specific people or just going on an indiscriminate shooting spree," Reno police Deputy Chief Tom Robinson said.
Surviving Afghanistan, but not school
True to his character, Mike Landsberry rushed to help others when chaos erupted.
The retired Marine, a popular math teacher at Sparks Middle School, tried to help when the two wounded students were shot.
A witness told the Reno Gazette-Journal that Landsberry was trying to intervene when the shooter killed him.
"That was the kind of person that Michael was," his brother, Reggie Landsberry, told CNN. "He was the kind of person that if somebody needed help, he would be there."
Sparks Mayor Geno Martini talked about the irony.
"It's very unfortunate that (the life of) someone like that, who protected our country over there and came back alive ... had to be taken at his work, at a school," he said.
Landsberry joined the Marine Corps in 1986, attained the rank of corporal and served as a field wireman, Marine spokeswoman Maj. Shawn Haney said.
On his class website, the teacher posted pictures of himself hiking in the wilderness and standing with a weapon beside an armored vehicle.
"One of my goals is to earn your respect while you earn mine," he wrote in a message to students. "I believe that with mutual respect that the classroom environment will run smoothly."
A Facebook memorial page for the teacher had more than 10,000 "likes" by early Tuesday. Thousands more honored him on a "Rest Easy Mr. Landsberry" page.
Both of the wounded students were hospitalized in stable condition Monday night, Sparks Deputy Chief Tom Miller said.
They were shot in the cafeteria and in a hallway. One was shot in the stomach, the other in the shoulder, Washoe County School District Police Chief Mike Mieras said.
Authorities have not released the wounded boys' names.
Returning to a national debate
Sparks Middle School will be closed for the rest of the week as the shooting reignites the national debate over gun violence and school safety.
Last week, a student at an Austin, Texas, high school killed himself in front of other students.
In August, a student at a high school in Winston-Salem, North Carolina, shot and wounded another student in the neck.
Another shooting took place at an Atlanta middle school in January, though no one was hit.
That same month, a California high school student wounded two people, one seriously.
The Nevada shooting comes almost a year after a gunman killed 26 people at Sandy Hook Elementary School in Newtown, Connecticut, igniting nationwide debate over gun violence and school safety.
Since the Newtown shootings last December, proposed school security plans across the country have included arming teachers, adding armed security guards and bringing bulletproof backpacks and whiteboards.
Some teachers have even started taking self-defense and combat classes in case a shooter enters their school. One class teaches how to escape or take cover but focused most of its four hours on how to fight and disarm an attacker -- something few educators have ever considered how to do.
The mother of a student killed in Newtown said Monday's shooting reinforces the need to find solutions to keep students safe.
"The unthinkable has happened yet again, this time in Sparks, Nevada," Nicole Hockley said in a statement. " It's moments like this that demand that we unite as parents to find common sense solutions that keep our children -- all children -- safe, and prevent these tragedies from happening again and again."
But what those solutions are will remain fuel for perpetual debate.
CNN's Stephanie Elam, Evan Perez, Amanda Watts, Chuck Johnston and Kim Segal contributed to this report.