Colorado Police searched a high school room by room Thursday after a cook reported seeing two suspicious men in the building around dawn, both wearing camouflage and one in a ski mask.
Authorities said no classes or groups were meeting in Boulder High School at the time. Police sealed off the building, and school officials canceled classes for the day.
A four-hour search by 23 officers in three SWAT teams found no signs of a break-in, Police Chief Mark Beckner said. The search included utility-access tunnels under the building and at one point two officers could be seen on the roof, their rifles drawn.
Beckner said there was no indication the intruders were armed.