WASHINGTON — U.S. officials said Thursday evening they have "specific, credible but unconfirmed" information about a threat against the United States coinciding with the 10th anniversary of the September 11 terrorist attacks.

"We have received credible information very recently about a possible plot directed at the homeland that seems to be focused on New York and Washington, D.C.," a senior administration official told CNN Pentagon Correspondent Barbara Starr.

The plot is believed to involve three individuals who have possibly entered the United States, at least one believed to be a U.S. citizen, the official said.

U.S. officials believe the threat is a vehicle laden with explosives but "the intelligence picture is not completely formed," the official said. "Not enough is known about the potential operatives and their plotting."

The official said the information came in around noon Wednesday.

Officials said they were taking the threat seriously, while evidently trying to temper the news by saying such threats are commonplace during key events.

"It's accurate that there is specific, credible but unconfirmed threat information," said Matthew Chandler, spokesman for the Department of Homeland Security. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days. Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots under way.

"Regardless, we take all threat reporting seriously, and we have taken, and will continue to take, all steps necessary to mitigate any threats that arise. We continue to ask the American people to remain vigilant as we head into the weekend," Chandler said in a prepared statement.

A government official told CNN that members of Congress were briefed by White House, intelligence and other officials Thursday about the threat.

The lawmakers were told that officials are "strongly concerned" and "are not taking anything for granted," the source told CNN.

In addition, a senior administration official with first-hand knowledge told CNN National Security Contributor Fran Townsend that there is "specific, credible but unconfirmed threat information."

But the official added that there are a lot of questions about this information -- "weird things."

There are questions about sourcing and other points that need to be clarified, the official said, but because of the sensitivities about the coming 9/11 anniversary officials felt they should inform people.

Multiple senior counterterrorism officials told Townsend on Thursday evening that the information came in the previous 12 to 14 hours. Such last-minute information is inevitable, the sources said.

A U.S. intelligence official told CNN National Security Producer Pam Benson, "We're trying to sort out" if what they are seeing "is accurate."

"We can't tell if anything is going to happen, " said the official, who added, "This is a very sensitive period, which adds to the anxiety."

A federal law enforcement official told CNN National Correspondent Susan Candiotti that the administration is fully aware of a threat of an attack on the United States on or about September 11, and that "we're taking it very seriously." The official said the threat is not specific as in being detailed in nature.

"We're running it down vigorously," the official said. "The FBI, JTTF (Joint Terrorism Task Force), and others are fully aware of it."

The official added that the threat appears to go beyond intelligence gathered from Osama bin Laden's compound after he was killed by Navy SEALs. That information talked about aspirations to carry out an attack on U.S. interests on the 10th anniversary of the 9/11 attacks.

"Like any threat, we're taking it very seriously," the official said.