Story highlights Witness: Shooter yelled, "Why are you laughing at me?"

"There's a kid with a gun," student tells 911 dispatchers

Slain teacher served in Kuwait and Afghanistan with the Air National Guard

The two wounded students were in stable condition with non-life-threatening wounds, police say

"This is a student at Sparks Middle School. Can you please send police out here? There's a kid with a gun."

"Somebody brought a gun to school. They shot a teacher."

"I got a kid down who's been shot."

As police tried to piece together how and why a seventh-grader shot and killed a teacher and wounded two other students at his Nevada school, recordings of the first calls to police captured the horror and chaos he unleashed.

The 12-year-old boy, whose name has not been released, began by shooting a fellow student in the shoulder, police said Tuesday. Then he turned his gun on math teacher Mike Landsberry before shooting a second student in the abdomen, Washoe County School District Police Chief Mike Mieras said.

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After that, he shot himself to death with his pistol, which Sparks Deputy Police Chief Tom Miller identified as a Ruger 9mm semiautomatic.

"We got a guy with a gun. He's down from a head shot wound. Could be our shooter," one of the responding officers is heard telling dispatchers on the 911 calls. "He's out there on the basketball court."

Landsberry walked toward the shooter on a playground basketball court after the first student was hit, saving lives, according to authorities.

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"Mr. Landsberry's heroic actions, by stepping toward the shooter, allowed time for other students in the playground area to flee," Mieras said.

Despite previous reports indicating the two wounded students had been shot inside the school building, all the shots were fired outside, police 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'

Authorities haven't said why they believe the boy opened fire.

Many have speculated that bullying might have played a role, 13-year-old Kyle Nucum told CNN's "The Lead."

That could be the case, said Nucum, a student at Sparks Middle School who didn't know the shooter but ran for cover after seeing him shoot Landsberry. As he fled, he heard the shooter shouting.

"He was yelling a bunch of things while we were running," Nucum said. "He was yelling stuff like, 'Why are you laughing at me? Why are you doing this to me?'"

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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 a bad day."

But for whatever reason, the boy 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 the boy's 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 the shots erupted.

"That was the kind of person that Michael was," said his brother, Reggie Landsberry. "He was the kind of person that if somebody needed help, he would be there."

Landsberry was an Alabama native who graduated from high school in Reno, next door to Sparks, in 1986. After his stint in the Marine Corps, he got an education degree from the University of Nevada in Reno. He joined the Air National Guard in 2001, rising to the rank of master sergeant and serving as a cargo specialist in Kuwait and Afghanistan, the Guard said.

Returning to a national debate

The Nevada shooting comes almost a year after a gunman killed 26 people at Sandy Hook Elementary School in Newtown, Connecticut, igniting a 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 white boards.

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 focuses most of its four hours on how to fight and disarm an attacker -- something few educators have ever considered how to do.

Meanwhile, reports of school violence have continued.

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-area middle school in January, though no one was hit.

That same month, a California high school student wounded two people, one seriously.

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.