(CNN) -- "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.
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.
"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.
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."
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.