What authorities initially said was an explosive device being found on the playground of a Montana elementary school turned out to be a harmless plastic bottle wrapped in tape.

The item was found just before 8:30 a.m. Tuesday at Rossiter Elementary School in Helena, and prompted an evacuation.

The Lewis and Clark County Sheriff's Office initially said the item was an improvised explosive device and that it had detonated before authorities arrived. In an update Tuesday night, the sheriff's office said an investigation determined that it was not an explosive.

"The suspected device was a plastic bottle with tape and had debris around it that included nuts, bolts and washers and an unidentifiable liquid inside and around it," the department said in a Facebook post. "Preliminary findings show no powder or chemical residue and the fluid is non-flammable."

Rossiter Elementary school. Google

Sheriff Leo Dutton told NBC affiliate KTVH in Helena that they believe a homeless man removed the bottle from a construction site and left it on the playground.

"He didn't do anything wrong," the sheriff said. "He wasn't trying to plant a bomb; he didn't even think it was that."

Earlier Tuesday, Dutton told The Associated Press that no threat was against the school prior to the item being found. The county's undersheriff, Jason Grimmis, told reporters that they are reviewing surveillance video in the area.

The sheriff's office said all East Helena Schools and Helena District 1 Schools were on lockdown Tuesday as authorities searched the buildings. The schools were cleared and the lockdown was lifted a few hours later.

The state Highway Patrol, the FBI, and the Bureau of Alcohol, Tobacco, Firearms and Explosives assisted in the investigation.