No injuries have been reported.
Students were evacuated from the school after authorities made sure the path was clear of other devices.
Students are being bussed to a nearby school where they can be picked up by their parents.
"I didn't get too close of a look because once they came over, I just barely noticed it when they came over, and then they said that we had to get away from that area, just in case", Lee said.
In a Facebook post, East Helena School District 9 said: 'After an exhaustive search of the school grounds in East Helena by the East Helena Police and the Lewis & Clark County Sheriff's Office.
Dutton says it's not clear exactly when the device exploded, but that no children were around when it did.
Dutton says they don't have a suspect or a motive for the explosion.
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All schools in the state's capital city, as well as East Helena, were initially locked down after the incident as authorities searched those locations.
There were no threats made against the elementary school or other campuses before the explosion.
Officials say they don't know for certain when the bomb was detonated but believed it happened sometime Tuesday morning.
A homeless person found the bottle at a construction site and left it at the playground, Dutton said.
Sierra Rd. has been closed between N. Montana and Frontage Road. It has not been determined yet if students will be allowed to return to class on Wednesday. Parents are being asked to pick up students the Little Red School House.
Agents with the FBI and the ATF (Bureau of Alcohol, Tobacco, Firearms and Explosives) attended the scene to ensure that the area is safe.
This is a developing story.