Police launched a room-to-room search of Boulder High School early Thursday after a cook reported seeing two suspicious men in the building, both wearing camouflage and one wearing a ski mask.

Authorities said no classes or groups were meeting in the building at the time and the school was locked down. The initial search with a bomb-sniffing dog turned up nothing, but a second, more thorough search by a SWAT team was planned, city spokeswoman Julie Brooks said.

Police Chief Mark Beckner said there was no indication that the men were armed.

"This could be a spring, end-of-school-year prank. It could be a burglary. It could be kids goofing around, or it could be more serious," Beckner said. "Given the times that we live in and recent events, we have to take all precautions."

Classes were canceled for the day. About 50 students who were already on school buses were taken to another school, where their parents were told they could be picked up, district spokesman Briggs Gamblin said.

Outside a nearby snowboard shop, Boulder High freshman Jake Hanifin, 15, and his friends said the incident didn't sound like a prank.

"I think it's hard to believe it's a joke if a kid comes in with a ski mask," he said. "To push a joke to that limit would be pretty intense."

The cook had spotted the two men around 6 a.m. on the first floor of the three-story building, and they ran when she yelled at them, Beckner said. He said the cook described them as young but said she didn't get a good look at their faces.

No threats had been made against the 1,900-student school in recent days, the police chief said. Earlier this spring, graffiti had been found inside the building saying people would die on April 19, the anniversary of the Columbine High School shootings near Denver. Classes were held that day with extra security, he said.