Shots were fired outside the U.S. Capitol Thursday afternoon by an unknown gunman, and at least one Capitol Police officer was injured in the incident, police said.

The shooting, at about 2:19 p.m., sent the busy Capitol complex into lockdown, with legislators and other employees told to shelter in place. It appeared to have taken place to the north of the Capitol itself, in the direction of the Senate’s large office buildings.

Emergency vehicles clustered at the normally busy three-way intersection of Constitution Avenue, 2nd Street, and Maryland Avenue NE. After the first shots were heard, officers with semi-automatic weapons were seen running toward the building’s exits.