.
It's unclear at this time what the girl's motive was or how she got the gun.
"We don’t have a lot of details at this time of 'why' — that is being investigated," Anderson said. "We’re following all leads."
Authorities have not released the girl's name but did say she is from nearby Idaho Falls.
Police were called to the scene on reports of shots fired around 9.15am.
The school district's superintendent Chad Martin called the day's events a "nightmare."
"Today we had the worst nightmare a school district could encounter. We had a school shooting here at Rigby Middle School," he said.
"What we know so far is the shooter has been apprehended. There is no further threat to the students."
He added that schools would be closed district-wide, but that counselors would be on hand for students who need them beginning Friday morning.
The adult who was shot was treated for a bullet wound at the hospital and released Thursday, Dr Michael Lemon, trauma medical director at Eastern Idaho Regional Medical Center, told reporters during a press conference.
He said the students who were shot were being held overnight at the hospital, and are both considered in fair condition.
One of the students may need surgery, he said.
"It’s an absolute blessing" that they weren’t hurt worse, Dr Lemon said.
After the reports of shots fired at the school, multiple law enforcement agencies responded, and students were evacuated to a nearby high school to be reunited with their parents.
Another student, Lucy Long, told the Post Register newspaper in Idaho Falls that her classroom went into lockdown after they heard gunshots, with lights and computers turned off and students lined up against the wall.
Lucy comforted her friends and began recording on her phone, so police would know what happened if the shooter came in.
The audio contained mostly whispers, with one sentence audible: “It’s real,” one student said.