The headquarters of the Republican National Committee was placed under lockdown after "vials of blood" were reportedly sent in a package.
A large police presence could be seen on Capitol Hill while hazmat teams attended to the scene.
The reason for the evacuation remains unclear, however unconfirmed reports suggest "blood was sent in a package and something broke open", according to the New York Post.
Police were said to have told politicians that they were investigating a "suspicious substance" at the address of the Republican National Committee.
A notice issued by the police stated: "Staff and other personnel are directed to AVOID THIS AREA until further notice."
The bomb squad were also thought to have attended the scene.
Capitol Police issued an "all clear" at the RNC after investigating the building, it has been reporter.ABC reporter, John R Parkinson wrote on social media: "USCP tells me the RNC is “all clear” after investigating a suspicious substance there.
"Another officer told me 'nobody is messing around' after multiple pipe bombs were placed on Capitol Hill in January 2021."