She has reportedly drawn a handgun from her backpack and fired multiple rounds, both inside and outside the premises of Rigby Middle School.
Two students and a staff member were wounded in the process.
The girl was then disarmed of her handgun afterwards by a female teacher and held until law enforcement officers arrived on the scene, U.S. authorities added.
Shooter is between 11-12
The girl, who is said to be between 11-12 years old, has since been taken into police custody.
AP News reported that she was from the nearby city of Idaho Falls.
However, it is unclear if she is also a student at the school.
Her motive for the attack and where she got the gun are currently under investigation.
As for the three victims, they are expected to survive, with their injuries reported to be non-life-threatening, according to British media The Independent.
Students fearful when they realised they were not experiencing a school shooting drill
A seventh-grade student, Madison Jensen, said that fear spread among the school's students during the shooting, once they realised that they were not experiencing a school shooting drill.
Jensen was quoted as saying:
"We didn't think anything of it at first, until we heard footsteps in the hallway and then we heard a police siren ... and that's when we started realising it was real."
Another seventh-grade student, Maren King, said that she thought the noises were initially part of the principal's attempt to make drill seem more realistic.
She added that once students realised it was real:
"We were freaking out — a few kids were crying. Some kid was sitting next to me. I was trying to rub his back, make him feel better."
Counsellors to be made available for students
The Superintendent of the school's district, Chad Martin, has since announced that all schools will be closed districtwide, so that students can be with their parents.
Martin also added that counsellors would be made available for the students, beginning from Friday, May 7.
AP News further quoted him as saying, "This is the worst nightmare a school district could ever face. We prepare for it, but you’re never truly prepared."
In the meantime, a prosecutor for the county, where the school is located, said that while criminal charges will not be made until investigations are completed, it is possible that three counts of attempted murder might be included.
Top image screenshot from Idaho Local News 8 YouTube