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.

A senior administration official told: “The president was briefed. He directed that the intelligence community and the administration take all necessary steps to ensure vigilance.”

Earlier Thursday, Homeland Security Secretary Janet Napolitano told reporters that intelligence officials had picked up “lots of chatter” on jihadi websites and elsewhere about the impending 10th anniversary of the September 11 terrorist attacks “and we’re taking it all seriously,” but nothing yet that warranted issuing a threat advisory.

Nonetheless, the department will be at a heightened level of readiness as the nation commemorates the anniversary, “staffing up” the Federal Air Marshals Service and other agencies, she said.

“We don’t have something that would reach that standard (of a credible threat), but we still have lots of chatter out there. And we take every bit of that seriously and are tracking it down,” Napolitano said. “In the intel world, there’s lots of chatter, and we’re taking it all seriously. And should there be something that rises to the level where I have to issue a threat advisory, we will issue a threat advisory.”

A federal official told CNN’s Mike Ahlers that Napolitano was aware of the threat when she made those remarks.

Napolitano said it is not uncommon to see increased chatter before major events. “We know it’s an iconic day to al Qaeda, in part because of what was found at the (Osama bin Laden) compound. So we are preparing accordingly,” she said.

“I don’t want to give those [details] out because I don’t want to tell the bad guys exactly what we’re doing,” Napolitano said. “But I think it’s fair to say that in addition to asking citizens to be vigilant and so forth, that we have ourselves leaned forward and have made sure that we are doing all that we can from the DHS perspective.”

One U.S. official told CNN that al Qaeda “has long been focused on commemorative dates for attacks, to include the anniversary of 9/11.”

“U.S. counterterrorism entities have been alert for any leads, and for the first time we’ve received specific and credible – but unconfirmed – threat information linked to the 9/11 anniversary,” the official said. “Any and all leads related to possible plots are of course being run to ground.”

The official said the threat is based on one report. “It has a date and a few details, which makes it specific and credible,” said the official, “but that doesn’t mean we are going to be attacked. … It’s not necessarily true, however we know there is interest in an attack around 9/11.”

CNN’s Lesa Jansen, Mike Ahlers and Jessica Yellin also contributed to this report