Police have been searching a school in the US state of Colorado after a cook reported seeing two suspicious men, one in a ski mask and both in camouflage.
Nothing has yet been found but Boulder High School remains closed to pupils, Boulder police chief Mark Beckner said.
He said the incident could turn out to be a prank or a burglary, but stressed that the police were taking no chances.
The alert comes less than a month after the Virginia Tech campus shooting in which 32 people and the gunman died.
"Given the times that we live in and recent events that we've had, we have to take all precautions," Mr Beckner said.
He said the canteen worker had spotted the two suspiciously-dressed young men just after 0600 (1200 GMT), before staff and pupils arrived.
They had run off after being challenged by the cook, he said. No weapons had been seen.
The 1,900-student school was expected to remain closed for the rest of the day while officers searched the premises and grounds.
A bomb-detection dog had also been brought in but had found nothing, Mr Beckner said.
Eight years ago, Colorado was the scene of one of America's most infamous school killings, when two students at Columbine High School killed 12 classmates and a teacher before shooting themselves.