D.C.'s chief of police is urging people not to change their weekend plans because of a new terror threat.

The U.S. government received a credible but unconfirmed terror threat ahead of the 10th anniversary of the Sept. 11, 2001, terrorist attacks.

"I feel very comfortable that we have a very safe environment for the weekend," Lanier said. "I want people to go out and do what they were going to do without this threat stream being present, because when we start modifying our behavior and people don’t go out in public, we’ve lost this fight already."

Officials also have said that everyone must remain vigilant and report any potential suspicious activity or items they see.

All Sept. 11 commemorative events will go on as planned, Lanier said.



The threat comes from a tribal area of Pakistan and involves car bombs and truck bombs detonating in New York and D.C. on Sunday, NBC News's Pete Williams reported. It also mentioned bridges and tunnels, according to Jonathan Dienst, of WNBC.

The information was received Wednesday night. The president was briefed Thursday morning and again throughout the day, Williams said.

The intelligence source said two or three men could be coming to the U.S. for the plot, and two might be Americans flying in from Dubai, MSNBC.com reported. Partial names common in the Middle East were given.

Counter-terrorism agencies are taking it very seriously, a U.S. official said.

"We don't take this threat lightly. We take it very seriously, but we are ready for it,'' said James McJunkin, assistant director in charge of the FBI's Washington field office.

The information was specific enough to warrant an advisory to the American people, but it’s unclear whether the plot is about to unfold, the official said.



"Al Qaeda and its affiliates have shown interest in milestone events and dates," the U.S. official said, but the official did not specify who is behind this particular plot.

"In this instance, it's accurate that there is specific, credible but unconfirmed threat information," read a Department of Homeland Security statement. "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."

Law enforcement officials were pursuing three people who may be traveling to the U.S. or who have recently entered the country, based on the detailed information received by the U.S. intelligence community late Wednesday, officials said. The intelligence suggested that al-Qaida planned to car bomb one of the two cities that were hit 10 years ago.

Vice President Joe Biden said Friday that there was no confirmation that anyone had traveled into the U.S. for such a plot, although the tip came from a credible source. "There's no certitude,'' he said.

"The thing we are all most worried about is what they call a 'lone ranger,' a lone actor, not some extremely complicated plan like it took to take down the World Trade towers,'' said Biden, who

appeared on the trio of network morning TV shows Friday.

A U.S. official said the source of the terror tip indicated that al-Qaida's new leader, Ayman al-Zawahri, was involved in planning the plot. But the official also said that many in the intelligence

community question that and other aspects of the source's information.

McJunkin said Thursday evening that his agents aren't seeking any particular individuals.



“There's no named individual,” he told reporters in a late-night news conference.



No terror warning has been activated.

The federal government is coordinating with state and local agencies to advise people of the potential threat. Virtually all area jurisdictions had police out in full force Friday.

Metro Transit Police said people should expect the heightened police presence, complete with K-9 units and security checks, to continue through the weekend.

On the National Mall this weekend, where the Black Family Reunion will be held, uniformed and plainclothes police will patrol.

D.C. Mayor Vincent Gray said that the District activated its plan Sept. 1 in preparation for the 9/11 anniversary.

“We take these threats seriously -- as we do all threats to our city -- and citizens should know we are taking all the appropriate steps to ensure their safety,” Gray said.

“I am asking the community to support our public-safety efforts by being alert to suspicious activity as well,” Gray continued. “Often people are hesitant to report something suspicious, but police say it’s better to err on the side of safety and report it to 911. Within the District, we also have a non-emergency call center at 311. We are prepared for larger-than-normal call volume on both lines.”

In D.C. Thursday afternoon, police officers were told not to go home early and that overtime might be canceled. Chief Lanier told The Associated Press that every one of her 3,800 officers would work at some point during remembrances.



"You’ll see mass transit, you’ll see restaurants, hotels, sporting events -- any place where there’s a crowd, we’re going to have an increased presence," she said.



All police officers will be working 12-hour days indefinitely, the AP reported. Unattended cars parked in odd locations risk being towed in the next few days, Lanier said.

The scheduling changes were “part of our plan” and “maintaining a certain sense of unpredictability is essential to the success of any security plan,” read a statement from the chief.



D.C. officials said that they aren't planning to cancel any events over the weekend.

Security already was heightened because of the 9/11 anniversary.

The security in place this weekend has been necessary for at least the past 10 years, Lanier said. In D.C., suspicious packages and vehicles are reported and searched constantly throughout the city -- evidence of a heightened public awareness produced at least in part by the terrorist attacks.



“It's not a good idea for everybody to relax after this anniversary is over,” Lanier said. “This is a way of life for us now.”



People walking or traveling near the U.S. Capitol will see more uniformed and plainclothes officers, additional police cruisers and may notice a bomb squad or SWAT team in areas of Capitol Hill, where they wouldn’t ordinarily be seen, said Sgt. Kimberly Schneider, a U.S. Capitol Police spokeswoman. The U.S. Park Police also is on alert, MSNBC reported.



"We’re well aware that this anniversary does bring with it a lot of emotion and concerns, but we’re certainly addressing it," Park Police spokesman David Schlosser said.

The mayor's office provided the following examples of reasons to call 911: