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 start Tuesday, wounding two, authorities in Roswell, New Mexico, said.

A girl, 13, was in stable condition Tuesday night following surgery, authorities said. A boy, 11, was in critical condition after surgery.

The horror might have been much worse if not for one staff member. “(He) walked right up to him and asked him to put down the firearm,” said New Mexico Gov. Susana Martinez.

Authorities detained a seventh-grader who is “responsible for this,” said State Police Chief Pete Kassetas.

Investigators didn’t yet have a motive for the shooting, Kassetas added.

State police planned to interview 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.