The Air Force Academy closed its south gate Friday morning for three hours after a scanner triggered a false alarm. The gate was reopened shortly before 9:30 a.m.
The south gate, off Academy Boulevard, is the academy's busiest, and is the main portal for commercial traffic.
At about 6:30 a.m., a dump truck passing through the gate triggered an alarm for suspected nitroglycerin. Security forces from the academy and an explosives detection team were called and the gate was closed, with traffic routed to the academy's northern portal, officials said.
All vehicles passing through the gate are subjected to inspections. For commercial vehicles, the gate has an extensive inspection regime that screens for explosives, chemicals and radioactive materials.
A thorough check of the dump truck revealed no explosives, an academy spokesman said. Almost all similar past incidents at the academy have been false alarms.