BREAKING: Senate buildings on lockdown over report of possible active shooter

"Our officers are searching in and around the Senate Office Buildings in response to a concerning 911 call. Please stay away from the area as we are still investigating. We will continue to communicate with the public here," the Capitol Police wrote.

"If you are inside the Senate Buildings, everyone inside should be sheltering in place as the report was for a possible active shooter. It should be noted that we do not have any confirmed reports of gunshots," police added.

Senate staffers reportedly received a message warning about an "internal security threat," telling those inside the building to move inside the nearest office, close and lock external windows and doors, and hide if out in a public space.

The incident at Russell Senate Office Building is likely a "mental health" situation, not an active shooter one, DC Fire and EMS told Fox reporter Adam Sabes.

Just after 4 pm, Capitol Police gave the all clear.

This is a breaking story and will be updated.