Dutton said school officials called deputies around 8:20 a.m. Tuesday after they found the remnants of the device, which had been crafted from a soda bottle, duct tape and other components.
No students or faculty members were nearby when the explosion happened and no injuries were reported, deputies said. No property damage was reported either.
Dutton told The Associated Press no threat had been made against Rossiter Elementary. Authorities evacuated students from the area Tuesday morning and announced Rossiter Elementary would be closed for the day as officials investigated.
Authorities issued a district-wide lockdown after the device was discovered Tuesday to allow officials to search the schools. The lockdown was lifted Tuesday afternoon.
Officials with the Lewis and Clark County Sheriff's Office bomb squad, Helena Police Department, Montana Highway Patrol, FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives were on the scene Tuesday morning.
Authorities continue to investigate.