(CNN) - The 12-year-old boy who killed a teacher and wounded two students began by shooting a fellow student in the shoulder, police said Tuesday.
The boy, whose name has not been released, then shot and killed teacher Mike Landsberry before shooting a second student in the abdomen, Washoe County School District Police Chief Mike Mieras said at a news conference.
The boy used a Ruger 9 mm semiautomatic handgun, said Sparks Deputy Police Chief Tom Miller.
Despite previous reports indicating the two wounded students had been shot inside the school building, all of the shots were fired outside, police said.
Landsberry walked toward the shooter on a playground basketball court after the first student was hit, saving lives, according to authorities.
"Mr. Landsberry's heroic actions, by stepping toward the shooter, allowed time for other students in the playground area to flee," Mieras said.
Miller, who said earlier on CNN's "New Day" that it wasn't yet clear if the boy was targeting specific people, declined to identify the boy out of respect for his family.
"They are grieving parents and are going through" a challenging, difficult time, Miller said.
'I think he took out his bullying'
Before Monday morning, the boy 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. Miller said Tuesday that authorities aren't positive where the gun came from, but believe it belonged to his parents.
Amaya said she thought the two wounded students were friends of the shooter.
They were in stable condition with non-life-threatening injuries Monday night, Miller said.
Authorities have not released the wounded boys' names.
Investigators haven't determined what prompted the shooting. Miller said only that police are exploring all avenues.
Amaya said she "saw him getting bullied a couple of times, and I think he took out his bullying."
Surviving Afghanistan, but not school
The teacher who died, Mike Landsberry, appeared to be trying to stop the incident when he was shot dead, Miller said Tuesday.
"It almost appears like he tried to talk him down," he said.
True to his character, the former Marine, a popular math teacher at Sparks Middle School, rushed to help others when chaos erupted.
"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."
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.
Returning to a national debate
The 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 in bulletproof backpacks and whiteboards.
Some teachers have 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 written 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 ongoing debate.