PORTSMOUTH, N.H. (AP) — Pease Air National Guard Base in New Hampshire was locked down for an hour Tuesday after reports of gunshots, but the alert was lifted after nothing was found amiss, and it was later determined no shots were fired, officials said.

Reports of what sounded like shots fired originated from the Guard’s headquarters building at the base in Portsmouth. The building is just inside the main gate.

“We were able to confirm there were no gunshots,” spokesman Lt. Col. Greg Heilshorn said. “There was no gun fired in the building or around the premises.”

Police are investigating what caused the noise.

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“Treating it with the utmost caution, we responded as if it were an active shooter scenario,” Heilshorn said. “Local law enforcement responded and swept the base building by building.”

The lockdown started at 11:22 a.m., and the base issued an all-clear announcement over its loudspeaker system at 12:37 p.m., The Portsmouth Herald reported.

There are no reports of injuries, New Hampshire Air National Guard Col. Bill Davis said.