Authorities said no one was injured when an already playground shortly before classes started.

An “improvised explosive device” was found in the playground of an elementary school in Montana in the United States, authorities said.

No one was injured following the device’s discovery on Tuesday at the Rossiter Elementary school in the state’s capital, Helena, which has population of about 30,000.

The device, which was found in the playground of the school, located in a residential neighbourhood just north of the city’s centre, had already exploded before students or faculty arrived at school, authorities said.

Undersheriff Jason Grimmis said the device included a soda bottle wrapped with duct tape.

“We don’t know what time this thing exploded,” Grimmis said, according to the Independent Record news site. “But what I do know is it did not explode while there was students there, while there was faculty there, which is a good sign.”

About 490 students were evacuated after school officials discovered the item shortly before classes began at 8:20am (14:20GMT). The school officials initially blocked off the area and called the police.

The Sheriff of the county, Leo Dutton, told The Associated Press news agency that no threats had been made to the school prior to the incident.

No suspect or motive was immediately identified.

“There’s a lot of unanswered questions,” Dutton said. “We’re going through it methodically and slowly, so we don’t miss something.”

All schools in the Helena and East Helena districts were briefly placed on lockdown as they were searched by law enforcement Tuesday morning. They were all cleared of danger shortly after.

Dutton said the state Capitol and government buildings were also searched as a precaution.

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