Top Congressional security officials are urging Members and staff to remain calm but vigilant in the face of a publicly announced terrorist threat.

The House and Senate Sergeants-at-Arms today reached out to their chambers in response to Homeland Security Secretary Janet Napolitano’s disclosure of a “recent specific, credible but unconfirmed threat” as the country prepares to mark the 10th anniversary of the Sept. 11 terrorist attacks.

Citing anonymous U.S. counterterrorism officials, the Associated Press reported that the threat involves a plot to detonate a car bomb at bridges or tunnels in New York City or Washington, D.C.

The Sergeants-at-Arms said staff should not alter their daily routines and that security will be ramped up around the Capitol for Sunday’s anniversary.

Senate Sergeant-at-Arms Terrance Gainer wrote on his blog, which is not accessible to the public, that the announcement of the threat was “rather confusing” and “well-intentioned, perhaps helpful, but not very well-coordinated.”

“Think of the information as coming from a person you know relatively well and trust, but the source of that ‘friend’s’ information is from a person about whom you know little or nothing,” Gainer wrote. “The ‘dot connectors’ are trying to unravel all sorts of gathered information. That is a good thing.”

Gainer, who is also chairman of the Capitol Police Board, said he has coordinated with Capitol Police Chief Phillip Morse, met with Secret Service Director Mark Sullivan and spoken with FBI officials to intensify security efforts that were already under way.

“Frankly, the threat is not unexpected considering what has been learned in the past few months,” Gainer wrote. “The whiff of a threat to attack U.S. interests on the 10th anniversary of the 9/11 attacks has been floated. The Capitol Police had already implemented enhanced security efforts in anticipation of potential threats associated with the 9/11 anniversary.”

Those measures went into effect Sept. 6, according to an email sent to House staff by House Sergeant-at-Arms Bill Livingood. They include deployment of additional vehicles and a higher level of patrol activity.

The efforts could be intensified further, he said, adding that he has been in “constant contact” with the intelligence community and the Department of Homeland Security to monitor the situation.

“Should the situation warrant, this office and the U.S. Capitol Police are prepared to take additional security measures, as necessary,” he wrote. “Be prepared to respond with patience when encountering U.S. Capitol Police officers who will be enacting security measures that may alter what is routine or normal.”

The officials urged staff to follow security procedures and contact Capitol Police if they notice any suspicious activity.