The US Capitol was put on lockdown after a woman reportedly tried to enter the building with a gun.
Police in Washington have said an "event" was underway shortly after 9am, and authorities were hunting a person.
A senate aide said that officers were looking for a woman who was "possibly armed" and who could be in the basement of the building.
The Senate seargant at arms issued a lockdown alert for the building and visitor center, although office buildings remained open.
One officer outside the Capitol said police were responding to "sketchy reports" of a woman who may have a gun.
The Senate sergeant at arms later said the lockdown had concluded.
Police gave an all-clear notice to reopen the Capitol building shortly after 9.30am, about a half an hour after the lockdown began.
It was not immediately clear whether the police had found the person they were seeking.
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