Update: The lockdown has been lifted, and the shooter identified as Larry Dawson of Tennessee. CNN reports that a police officer was not shot, and that a female civilian has been injured. A male suspect is in custody, according to the report.
Earlier: On Monday afternoon, Reuters and the Associated Press reported that shots had been fired near Capitol Hill. The person who fired shots is still unknown, though Reuters reported shortly thereafter that a police officer has been wounded by gunfire and that one person is in custody. The White House and U.S. Capitol were immediately placed on lockdown, in spite of the White House Easter Egg Roll event occurring on the White House's lawn.
According to a memo that had been sent out to House of Representatives staff, the Capitol is on lockdown, and everybody in the immediate vicinity was advised to take cover. "No one will be allowed to enter or exit any buildings," the memo reads. In the minutes after the suspect was taken into custody, the lockdown was not lifted.
Little is known about the person who is in custody, though CNN reports that they tried to enter the building and was apprehended. The Capitol visitors' center website was down as of 3:13 p.m. Immediate reports from the scene suggest that the police officer involved in the shooting is not critically wounded. NBC News reports that a person was being transported into an ambulance via stretcher, and that the ambulance has a police escort.