US Capitol, visitors center on lockdown with police investigating.

WASHINGTON -- The U.S. Capitol and the buildings within its complex are on lockdown with police investigating.

There were no immediate details Tuesday. It marks the second time in less than a week that the Capitol has been locked down.

Police have sent an email to congressional staff advising them to close and lock doors and stay away from external doors and windows.

Both the House and Senate were in session, with House members voting. In the Senate, lawmakers were delivering speeches from the floor.

Tourists sat in the visitors' gallery in the Senate, apparently unaware of what was transpiring.

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