A Capitol Hill police officer walks the grounds outside of the Capitol building on Thursday, Sept. 8, 2011, in Washington.

Washington, D.C. ? U.S. officials said Thursday they were investigating a credible but unconfirmed threat that al-Qaida was planning to use a car bomb to target bridges or tunnels in New York City or Washington to coincide with the 10th anniversary of the Sept. 11 attacks, the first tip of an “active plot” around that date.

The Homeland Security Department said the threat is credible and specific, but unconfirmed. The nation’s terror alert level has not changed, but raising it was under consideration Thursday night.

Law enforcement officials were investigating three people who recently entered the U.S. The threat was received by the U.S. intelligence community late Wednesday night, officials said.

“There is specific, credible but unconfirmed threat information,” said Janice Fedarcyk, the assistant director in charge of the FBI’s New York division. “As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days.”

Security has been enhanced around the country in the weeks leading up to the 10th anniversary. Law enforcement officials have been wary, particularly after information gleaned from Osama bin Laden’s compound in May indicated that al-Qaida had considered attacking the U.S. on the anniversary and other important dates.

The threat came in a single piece of information and was so specific — and came at such a time of already heightened alert — that it could not be ignored. The officials described the threat to The Associated Press on condition of anonymity to discuss sensitive security matters.

ABC News had initially reported that trucks missing from Kansas City, Mo., were possibly related to the threat, but have since withdrawn that statement after the trucks were found. CBS News reports that there are two distinct threats being investigated right now.

New York City Mayor Michael Bloomberg told reporters that police there were deploying additional resources around the city, but that New Yorkers should go about their business as usual. The city’s observance of the attacks will go on as planned, Bloomberg said.

The FBI and Homeland Security Department issued a joint intelligence bulletin Thursday night to law enforcement around the country urging them to maintain enhanced security and be on the lookout for suspicious activity.

Some District of Columbia police officers received an internal memo from department management on Thursday saying that the city had received “credible threats” and that additional staffing would be required, according to a law enforcement source who had seen the memo. Police in both cities were increasing their already beefed-up staffing levels in light of the recent intelligence.

President Barack Obama was briefed on the threat information Thursday morning and directed the counterterrorism community to redouble its efforts in response to the information, a White House official said.

White House officials said there were no plans to change Obama’s travel schedule on Sunday in light of the threat. The president is scheduled to mark the 9/11 anniversary with stops at New York’s ground zero, the Pentagon and Shanksville, Pa. He will also deliver remarks Sunday night at a memorial concert at the Kennedy Center in Washington.

Law enforcement officials are checking out all of the details included in the threat, said Rep. Peter King, R-N.Y.

“No need to panic,” King said. “They have not been able to confirm it yet.”

Thursday morning, Homeland Security Secretary Janet Napolitano told reporters that there was “a lot of chatter” around the anniversary of the attacks but that there was no information about a specific threat.