An improvised explosive device detonated at a Montana elementary school just before classes started Tuesday morning, authorities said, but no injuries were reported.

The Lewis and Clark County Sheriff's Office said in a statement that the device was found on the Rossiter Elementary School playground in Helena. No students were nearby when it detonated, officials said.

Authorities evacuated students from the school following the incident and locked down all Helena and East Helena schools as a precaution.

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“Our intentions are to make all the schools as safe as possible. We’ve been doing sweeps of the area, the playgrounds, the parking areas, and if the schools want us to come into the interior of the school, we’d be more than happy to oblige,” Undersheriff Jason Grimmis said at a press conference.

Authorities added that no threat was phoned in to the school or to local police.

“We want our bosses, the public, to know that things are under control, no one was hurt and we’re taking appropriate precautions,” Sheriff Leo Dutton said. “There’s a lot of unanswered questions. We’ll be looking for video, all kinds of things that will help us solve this.”

The FBI, Montana Highway Patrol, Helena Police Department and the Bureau of Alcohol, Tobacco, Firearms and Explosives are joining in the investigation.

Authorities said the bomb was in a soda bottle and wrapped with duct tape. They have also not ruled out the possibility of secondary explosives being placed elsewhere in the city. 

“If people are walking around and see a pop bottle with some tape on it, stay away from it and call it in,” Grimmis said.