BOULDER, Colo. -- Boulder High School was locked down before school started Thursday morning after two young men wearing camouflage and ski masks were spotted inside the school.

Classes for the day were cancelled but will resume Friday.

A cafeteria cook arriving at the school reported seeing the two suspicious people at about 6:30 a.m. in the cafeteria area, according to a school district spokesman. The worker did not see any weapons, police said.

The two school-aged men ran when the worker spotted them, according to school district spokesman Briggs Gamblin. He did not know if they ran out of the school or into another area of the school.

There were no students in the building at the time.

A woman told police that at 4:30 a.m., she saw three or four people on the bike path that goes behind the school property. She said the men were dressed in black and one of them had a ski mask on. She said they said hi to her and one of them even asked if she had dropped something on the path.

"We don't know if this is a prank, we don't know if this is a burglary, we don't know if it's something more than that," said Police Chief Mark Beckner. "You really can't take any chances these days."

Police immediately blocked off the entrances to the high school, and arriving students were not allowed to enter while the school search was being carried out. Police characterized the event as a suspicious incident and conducted several sweeps of the school.

Officers completed at least three searches of the school, including the underground tunnels, before they confirmed that the two men were not hiding inside the school.

The first search was preliminary. The second, more methodical search involved three teams of SWAT officers who swept through and checked inside all the rooms that had been locked, all the offices, closets and even the storage areas.

"That took quite a bit of time. After the school had been cleared initially, they took more time to go through every nook and cranny that they could find," said Beckner.

By mid-afternoon, officers went in a third time, conducting a walk-through with school administrators and employees to see if they recognized anything unusual.

There are no security cameras inside the school, according to Gamblin, but there about 15 cameras on the outside of the school. Police have looked at those surveillance videos but say the quality is not very good.

"You can't make identifications or even clothing descriptions," Beckner said.

However, he noted that an hour before the reported incident, two people were videotaped running eastbound from the school. But he doesn't know if that's related.

He said that there were no signs of a break-in and no alarms were tripped so it's not likely the men were in the school overnight.

A decision was made at 8:19 a.m. that the school would close for the day, according to Gamblin. Click here to read the news release from Boulder Valley Schools.

"We don't know if it was a threat at all. It was, again, two people dressed in camouflaged-type clothing. One with a ski mask. That normally raises some concern and alarm. Especially in today's environment, with what we've dealt with in this country. And that's what raises the concern. Whether they were an actual threat or not, there is no way for us to know at this time," Beckner said.

The University of Colorado temporarily closed 19 buildings on the nearby campus and notified key personnel of the incident. The buildings were reopened at noon. CU officials said no classes, finals, moving-out activities or graduation activities were interrupted.

One bus of about 50 Boulder High School students was already heading to the school when word came of the lockdown. Gamblin said that the bus was turned around and redirected to nearby Fairview High School. District officials said that parents were sent to Fairview to pick up children who had been on the bus.

Boulder High freshman Jake Hanifin, 15, 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 while talking with friends outside a snowboard shop a few blocks from the school. "To push a joke to that limit would be pretty intense."

Sophomore Sara Fleegler, 15, who went to the nearby downtown district with friends, said the scare was out of character for "such a relaxed place" as Boulder.

"It's just crazy that people could be in the school when it wasn't opened up yet," she said.

Boulder High School is located at 1604 Arapahoe Ave., and 1,900 students are currently enrolled.