Kassetas said the investigation was in the early stages, and authorities were still looking into a possible motive. He estimated more than 100 students and faculty members needed to be interviewed.
"I don't have a lot of answers as of yet," Kassetas said, adding police believe they have the only individual responsible for the shooting in custody.
"I want to make sure the community knows there's really nobody else out there," he said.
Kassetas said charges were pending against the boy but wouldn't say what authorities were considering filing.
A statement from the state police said authorities responded at 8:11 a.m. The school was placed on lockdown and children were bused to a nearby mall, where parents picked them up.
At the mall, parents waited anxiously. Some held hands, while others hugged each other.
Classes were later canceled and won't resume until Thursday.
Two prayer services were scheduled for Tuesday night at Roswell's Calvary Baptist and First Baptist churches. Pastor Chris Mullennix of Calvary Baptist said parents were worried and heartbroken, but there was a sense among many he spoke with that the community would be able to come together.
"This is something that strikes people to the core," he told The Associated Press. "We're not talking about a flesh wound or just a mental wound. We're talking about the very souls of people being shaken and rocked by something like this."
Mullennix said the prayer services will start the healing process.
"This is tragic but yet people in Roswell are tough, and people in Roswell will recover because we do have a sense of community, and I think that's really important," he said.
In the hours after the shooting, social media sites were flooded with sentiments offering prayers for the community. Some Berrendo students posted on their Facebook pages that they were frightened and didn't want to return to classes.