Police reopened Friday the seat of the US Congress in Washington after an emergency lockdown had been imposed due to a security alert.

Police reopened Friday the seat of the US Congress in Washington after an emergency lockdown had been imposed due to a security alert.

"The incident requiring staff to lockdown has concluded," U.S. Capitol police announced at about 9:45 a.m. "Staff may return to normal operations. Thank you for your cooperation."

Police had previously indicated that agents were searching for an individual, without providing further details, local media reported.

Republican Congressman Dennis Ross told the daily The Hill that an employee had been seen in the building wielding a firearm.

Soon after 9 a.m., cops sent a note telling Congressional staff to "close, lock and stay away from external doors and windows." The same orders applied to tourists at the Capitol Visitor Center.

Capitol police spokeswoman Eva Malecki said the lockdown had been ordered "out of an abundance of caution."

News of the closing of Congress came just as the details were announced about a sniper murdering five police officers in Dallas last night and wounding another nine people.