(CNN) -- A 12-year-old boy entered his middle school gym, pulled a shotgun out of a bag and opened fire on students waiting for school to begin Tuesday, wounding two of them, said authorities in Roswell, New Mexico.
A boy, 11, was in critical condition, and a girl, 13, was in serious condition, both with gunshot wounds, said Eric Finley, spokesman for University Medical Center in Lubbock, Texas. The boy was out of surgery, but the girl remained in surgery late Tuesday afternoon, Finley said.
A staff member at Berrendo Middle School disarmed the student after he "walked right up to him and asked him to put down the firearm," said New Mexico Gov. Susana Martinez.
Another school staff member received "very minor injuries" in the shooting and declined medical treatment so that he could continue to care for the pupils in the gym, the governor said.
The two student victims "were shot while simply sitting in their gym waiting to go to class, and one of them is in surgery," Martinez said at a news conference. "I'm asking that all New Mexicans, please, keep these two children in your prayers."
Authorities have in custody the lone student who's "responsible for this," said State Police Chief Pete Kassetas. The student who police say fired the shotgun is a seventh-grader, he said.
Investigators didn't yet have a motive for the shooting, Kassetas added.
Students had gathered in the gym to stay out of the cold weather before classrooms opened, the governor said.
A New Mexico state police lieutenant happened to be dropping off his child at the school when the principal saw him as she was locking the door during the shooting, Martinez said. She asked the lieutenant to assist, and he helped secure the scene, she said.
Said Chief Kassetas: "I commend the principal and the people that work at this school for saving many more lives."
State police were interviewing more than 100 students and faculty Tuesday. "We're going to be here for quite some time," the police chief told reporters Tuesday afternoon.
Investigators will examine "social media outlets and the forms of communication that students use," Kassetas said.
The state Department of Public Safety said in a news release that state and local police were called to the school at 8:11 a.m.
State Police public information officer Emmanuel Gutierrez said children were bused from the school after the threat had been "alleviated."
Police said the school was placed on lockdown after the shooting.
A vigil was scheduled at the Roswell Convention & Civic Center for Tuesday evening; classes at Berrendo were canceled for Wednesday, when counselors will be made available to the community, officials said.
"Today is the day that no superintendent wants to go through," Roswell Superintendent Tom Burris said. "This couldn't happen. It's unbelievable it could."