Nearly 800 terrified students and teachers were evacuated from a suburban elementary school after a caretaker found a pipe bomb on the roof.
Bomb disposal experts detonated the device using a high-pressure water cannon after it was found at Mountain View Elementary School in Layton, north of Salt Lake City, Utah.
The brave caretaker who spotted the bomb took it from the roof and then alerted the school principal, who called police, said school district spokesman Christopher Williams.
Precautions: A police officer searches the roof of Mountain View Elementary School yesterday after maintenance worker found a pipe bomb on the roof of the school and it had to be detonated by the bomb squad
'Why he did that and where he took it, we're not sure. He put himself in a dangerous situation,' he said.
No threats had been received by the school or any of the 85 other schools in the district, and no bombs have previously been found on district property, he added.
A different caretaker had been on the roof on Friday to retrieve a ball and had not seen any suspicious devices in the area where the bomb was found yesterday, said Mr Williams.
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'That doesn't mean it (the pipe bomb) wasn't up there. As to who may have placed it up there, we have no idea at this point,' he said.
The incident was being handled by the Layton police and fire departments and the Davis County Sheriff's Office bomb squad, which did not immediately return calls seeking comment.
The U.S. has been on a state of heightened alert since the terrorist attacks at the Boston Marathon last week, which killed three and hurt a further 180.
Just yesterday Canadian authorities revealed they had foiled an al-Qaeda-backed terrorist plot to derail a New York City-bound passenger train as it crossed the Niagara River.
Mountain View Elementary School: No threats had been received by the school or any of the other schools in the district, and no bombs have previously been found on district property
Bomb-sniffing dogs from nearby Hill Air Force Base scoured the school on Monday for further bombs but none were found.
Maintenance workers at all the district's schools were asked to inspect roofs as a precautionary measure, Mr Williams said.
'We drill a lot for fire, intruders, disasters. That means when there's a real situation like today, everything goes smoothly. Under the circumstances, things went as well as they could,' he said.