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       The United States Capitol Police (USCP) locked down or evacuated portions of the Capitol on Thursday morning to respond to reports of a suspicious vehicle near the Library of Congress in Washington, D.C., authorities said.

       The USCP called it an "active bomb threat investigation." A House Republican communications director on the scene confirmed to Fox News that Capitol Police have warned them about a man in the area of Independence Avenue and First Street who is "claiming to have explosives in his truck."

       According to the source, law enforcement have sent a negotiator to the area. The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) are responding.

       The USCP announced the suspicious activity on its Twitter page just after 9:40 a.m. local time and urged people to stay away from the area. Police are investigating reports of a possible explosive inside a truck in the area.

       Associated Press reporter Mike Balsamo tweeted that investigators "are trying to determine whether the suspicious device in the pickup truck is an operable explosive and whether the man in the truck was holding a detonator."

       Aaron Fritschner, communications director for Rep. Don Beyer, D-Va., shared on Twitter screenshots of message alerts he had received from the police agency. One message sent at 9:53 a.m. states, "Madison: Internal Security Threat: move inside office/lock doors, seek cover, and remain silent."

       A subsequent message said: "Jefferson: EVACUATE. Proceed to your designated assembly area. DO NOT exit out of the west side (First Street) of the building."

       Other areas in the region have also been ordered to evacuate to neighboring buildings, Fox News has learned, but the entire Capitol is not currently locked down.

       Police have said the investigation is ongoing. Meanwhile, many lawmakers are out of the area for their August recess.

       Fox News’ Tyler Olson and Kelly Phares contributed to this report.

       This is a developing news story. Please check back for updates.