Ohio's Wright-Patterson Air Force Base went into lockdown for several hours while security officials investigated reports of a shooter on the campus but eventually gave the all-clear.
The lockdown was repeatedly announced over loudspeakers on Thursday night at the base just east of Dayton. Stacey Geiger, spokeswoman for the base, said a report of a shooter came in at 9.25pm.
Over the next four hours, officials from the 88th Airbase Wing said responders conducted two sweeps of the National Air and Space Intelligence Centre, a three-story headquarters for the centre described as the Department of Defence's primary source for foreign air and space threat analysis.
The lockdown was lifted with the announcement of an all-clear around 1.40am on Friday. Base officials haven't commented on what prompted the report of a shooter or whether any shots were actually fired.
Spokespeople said they would provide more information when it's available. A media briefing was expected.
The base had said in an earlier tweet as the situation was developing that "All of our military and civilian employees are trained to quickly assess the situation and take appropriate action."