SEOUL, South Korea — The U.S. Osan Air Base in South Korea lifted a lockdown Monday that had been ordered as a precaution after someone reported an unscheduled “active shooter drill” at the high school. 

Security forces swept the school and its perimeters and found no injuries or suspicious activities, according to a posting on the base’s Facebook site. 

Morgan Nugent, principal of Osan American High School, said in a Facebook posting that officials decided to err “on the side of safety” and lock the school down after a teacher received what sounded like an automated call saying there was “an active shooter drill taking place.” There was no such drill scheduled Monday.

Osan Air Base, located south of Seoul, is the headquarters of U.S. air forces in South Korea, the 7th Air Force. The base is also home to the 7th Air Forces’ 51st Fighter Wing. About 7,500 to 8,000 U.S. troops, civilian employees and their family members live and work on base, according to the base’s public affairs office.

About 28,500 U.S. troops are stationed in South Korea as a deterrent to North Korea, whose 1950 attack started the Korean War.

The presence of the U.S. troops and annual U.S.-South Korean war drills are a continuing point of contention with North Korea, which claims that they are signs of hostility and designs at invading the North.