Law enforcement officials are warning people in the nation's capital to stay clear of Union Station because of police activity.

The Senate sergeant at arms tweeted Friday afternoon: "Due to police activity, remain clear of the area near Union Station. Please avoid this area until further notice."

Washington police tweeted that they were on the scene but said it's not an active shooter situation.

Union Station is a major train station and transportation hub in Washington. It is Amtrak's headquarters.

One ambulance was seen leaving the scene with its lights and sirens on, followed by a police car.

As they were elsewhere around the country Friday, flags on the towering poles outside the station's main entrance were flying at half-staff in remembrance of the terrorist attacks on Sept. 11, 2001.