The House went into recess just about the time the alert was issued. The Senate remained in session for a while afterward but eventually went into recess.
At 3 p.m., police lifted the restrictions with a message to staffers: The incident has concluded. It is now safe to resume normal activities.”
The White House also briefly went on lockdown, and the pedestrian area on Pennsylvania Avenue was closed for a short time.
The White House said President Obama was briefed on the incident.
• Tom Howell Jr., Ben Wolfgang, Jeffrey Anderson and Stephen Dinan contributed to this report.