HELENA, Mont. (AP) — An improvised explosive device blew up in an elementary school playground in Helena on Tuesday just before classes began, authorities said, causing no injuries but leaving investigators to puzzle over what was behind the explosion in Montana’s quiet capital city.

No threat was made against Rossiter Elementary school, Lewis and Clark County Sheriff Leo Dutton told The Associated Press.

Dutton said the device blew up at 8:21 a.m., just minutes before the morning bell rang to start classes. Dutton did not immediately answer whether children were in the vicinity or if the explosion damaged any property.

Officials planned a news conference Tuesday morning.

Students were evacuated from the school after authorities made sure the path was clear of other devices. The children were placed on buses and sent to another location where they could be picked up by their parents, Helena public school officials said.

All schools in Helena and East Helena were locked down and were being searched by officers. School officials said in a statement that the lockdown was a precautionary measure, and that no students would be released outside for recess.

The FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives and the Montana Highway Patrol were assisting sheriff’s and Helena police officials in the investigation.

Helena is a small city of about 30,000 people in the foothills of the Rocky Mountains. The school is in a residential neighborhood just north of the city’s center.