"We heard shots. They told us to get behind a car," Sen. Sherrod Brown, D-Ohio, said as he re-entered the Capitol building just moments before it was placed on lockdown.
There is "no information this is related to terrorism or this anything other than an isolated incident," Capitol Police Chief Kim Dine told reporters.
An initial alert to lawmakers and staffers came around 2:25 p.m. A half hour later at 2:55 p.m., the lockdown was lifted. The Supreme Court was also briefly closed following reports of gunfire.
"Close, lock and stay away from external doors and windows. Take annunciators, emergency supply kits and escape hoods; and move to your office's assigned shelter in place location or the innermost part of the office away from external doors or windows," Capitol Police told congressional staffers staffers in an email.
The Capitol this week is the center of a contentious political battle over an ongoing government shutdown.