"I saw him getting bullied a couple of times," student Amaya Newton, who witnessed the shooting at Sparks Middle School 5 miles east of Reno, told CNN Monday night.
"I think he took out his bullying on [the school]," she said.
Newton's mother, Tabitha Newton, agreed with her daughter's account, saying her daughter knew the unidentified shooter personally and had said he was regularly "getting picked on."
Michelle Hernandez, who also witnessed the shooting, told the Reno Gazette-Journal she heard the shooter ask: "Why you people making fun of me? Why you laughing at me?"
Officials did not immediately say what they thought prompted the shooting, which police said occurred about 7:15 a.m., just before the start of school.
Students were returning Monday from a weeklong fall break.
The Sparks school -- one of four middle schools in the city of 90,000 -- caters to seventh- and eighth-grade students between 12 and 14 years old.
The 13-year-old boy shot and killed 45-year-old eighth-grade math teacher and former Marine Corps member Michael Landsberry and wounded two other students before fatally shooting himself, Reno Police Deputy Chief Tom Robinson told reporters.
The wounded students were reported in stable condition late Monday.
Landsberry was shot after he told the boy to put down his gun, police said.
"To hear he was trying to protect those kids, that he stepped up and tried to stop the situation, doesn't surprise me at all," Landsberry sister-in-law Chanda Landsberry told the Gazette-Journal.
"He could have ducked and hid, but he didn't. That's not who he is. He was trained to help," said Landsberry, who is married to Michael Landsberry's younger brother, Reggie.
Michael Landsberry, who was also a member of the Nevada Air National Guard, leaves behind his wife and two step-daughters. He celebrated his wedding anniversary Oct. 18.
An interfaith candlelight vigil to honor the shooting victims is planned for 7 p.m. Wednesday.
More than two dozen school counselors and psychologists would be on hand at the school Tuesday to provide counseling services for students, staff members and families, officials said.
Amaya Newton told CNN the shooter "was really a nice kid."
"He would make you smile when you were having a bad day," she said. "He'd just ask you if he could buy you something. He was just really a nice kid."