School officials made the discovery shortly before classes began at Rossiter Elementary School. They blocked off the area and called police at about 8:20 a.m., said Lewis and Clark County Sheriff Leo Dutton.
An investigation found the plastic bottle wrapped in black electrical tape was full of washers, nuts and bolts, along with a non-flammable unidentified liquid, Dutton said. There was no detonator attached to the bottle.
A homeless person found the bottle near a construction site and left it in the playground, he said. No threat had been made against the school, and there were no injuries or property damage.
"It wasn't malicious," Dutton said. "It did look like a bomb, the school acted appropriately."
The 490 students walked to a nearby location where they could be picked up by their parents, said Superintendent Tyler Ream. School buses were brought in to keep them warm. Parents were notified via the school's messenger system.
Police closed the school and searched the grounds for additional devices, and thousands of students across Helena and East Helena were kept inside while authorities swept for bombs outside, authorities said.
Searches also were conducted at the state Capitol and government buildings, Dutton said.
Helena is a small city of about 30,000 people in the foothills of the Rocky Mountains. The school is in a neighborhood just north of the city's center.
This story clarifies that the students walked to the evacuation site.
This story clarifies that authorities don't know when the blast occurred, but that remnants from the explosion were found before classes began.
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