A police officer has died after slamming a vehicle into the security barrier surrounding the Capitol building, US media report.
The suspect is reported to have emerged from the vehicle brandishing a knife before being shot by a police officer.
The Capitol was locked down and a National Guard team was deployed as police instructed staff to remain indoors.
Two officers were struck by the vehicle and have been taken to hospital.
Footage of the scene showed a helicopter flying overhead and what appeared to be two people on stretchers being moved into ambulances.
Onlookers were told to clear the area.
The entry point on Constitution Avenue where the vehicle struck the barricade is directly where Senators and their staff come to the Capitol each day, according to observers.
But Congress is currently in recess, meaning the majority of politicians are not at the Capitol complex today.
President Joe Biden left Washington earlier in the day for Camp David, the presidential retreat in Maryland.
However some reporters, maintenance workers and Capitol Hill employees are likely still on the Capitol grounds.
????????A helicopter just landed on the east front of the Capitol. I’ve never seen anything like this before. ever. pic.twitter.com/LxV0mywRSe
— Jake Sherman (@JakeSherman) April 2, 2021
Shortly after 13:00 local time (18:00 GMT) the Capitol Police alert system sent an email to lawmakers and their staff ordering them to stay away from exterior windows and doors. Anyone outside was instructed to seek cover.
The FBI’s Washington Field Office is responding to the situation and is now providing support to the Capitol Police. The US Attorney General is also aware of the incident and is being updated, according to CBS News.
The incident comes nearly three months after the deadly 6 January riot at the Capitol.
