Gunshots have been heard at the US Capitol Visitors Centre, according to US Senate sources.
Workers in the Capitol were told to "shelter in place" and those outside were urged to take cover.
Some reports suggested that the gunman had been captured and one police officer had been wounded.
There were also reports that an individual had tried to gain entry into the White House, though it was unclear whether this was the same suspect.
A Capitol Police spokeswoman had no immediate comment but said the office would be issuing a statement soon.
The White House was initially put in lockdown, but US media reports suggested this was lifted after a matter of minutes.
A US Capitol police officer, who asked not to be identified, said that the gunman was being transported to hospital.
The officer did not identify or describe the suspect and he added that there were no additional suspects.
The event unfolded with Congress in recess and lawmakers back in their districts.
The shooting happened just a few hours after a drill for an active shooter took place at the Capitol, creating further confusion.
Cathryn Leff tweeted that she was at the visitor's center when she heard gunshots while going through a security check point.
That moment when it goes down ..everyone is screaming & running and you can't see where the #ShotsFired are from pic.twitter.com/ihfc01nmlt
— Cathryn Leff, LMFT (@Cathrynlefflmft) March 28, 2016
The Capitol Visitors Center is where tourists gather to take tours.
