Last Updated 11:42 p.m. ET
WASHINGTON - Government officials say at least two of the men suspected of being involved in a possible al Qaeda attack to coincide with the 10th anniversary of the 9/11 attacks are believed to be U.S. citizens or have U.S. traveling documents.
The officials say the information comes from a single source and it has not been confirmed.
One official says the would-be attackers' primary goal is to set off a car bomb, but if that proves impossible, they have been ordered to find some other way to cause violence. The tipster says the would-be attackers are of Arab descent and may speak Arabic as well as English.
The officials say the tips reached the CIA by way of a long-term source in the Afghanistan-Pakistan region who has proven reliable in the past. They spoke on condition of anonymity to discuss the investigation.
As reported, counterterrorism officials are chasing a credible but unconfirmed tip that al Qaeda has plans to set off a car bomb in New York City or Washington around the Sept. 11 anniversary, with bridges or tunnels as potential targets. It was the first word of a possible "active plot" timed to coincide with commemoration of the terror group's attacks a decade ago.
Counterterrorism officials were investigating the threat throughout the night and into Friday, as police in New York and Washington said they would increase their already stepped-up staffing levels in light of the recent intelligence.
The threat originated from Pakistan and what remains of al Qaeda's core leadership, including its new head Ayman al Zawahiri, reports CBS News homeland security correspondent Bob Orr. A U.S. official told Orr the original intelligence source provided enough specifics relating to locations, dates, methods and operatives to suggest that "it is more than 'aspirational.'"
The intelligence suggested that al Qaeda planned to car bomb one or both of the two cities that were hit 10 years ago.
A source told CBS News that another possible terror threat is also being investigated. Information on this secondary threat was obtained from a different overseas source than the most recent plot threat received over the last 48 hours. Authorities have not yet linked the two, and no specific target was provided. But the threats remain unconfirmed.
Orr said that 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.
CBS News also learned that the U.S. is continuing to coordinate with Pakistani Intelligence, following the arrest last week of al Qaeda senior operations leader Younis al Mauritani, one of the most wanted terrorists, by Pakistan's ISI in Quetta.
On CBS' "The Early Show," Vice President Joe Biden called the intel "a real threat," and said investigators were following every thread they could. "People should be alert, they should not alter what they're doing," he said, noting "we have significant security, local police and federal agencies working on this."
Police in New York and Washington increased their already stepped-up staffing levels. In New York City, authorities were stopping vehicles at the 59th Street Bridge Friday, causing a major traffic backup. National Guard troops and transit police carrying assault rifles watched the crowds at Penn Station.
White House officials said President Barack Obama had no plans to change his scheduled trips to New York's ground zero, the Pentagon and Shanksville, Pa., on Sunday to mark the anniversary.
This latest threat "should not surprise any of us," Secretary of State Hillary Rodham Clinton said during a speech in New York. Telling the public "is intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and the ears of vigilance," she said.
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.
At the Pentagon, officials said there have been no changes to military base security levels since they were upgraded earlier in the week, before the threat information came in. And there have been no changes to the schedules of Defense Secretary Leon Panetta and Joint Chiefs Chairman Adm. Mike Mullen, who will be attending 9/11 commemoration events throughout the weekend.
Security has been enhanced around the country in the weeks leading up to the 10th anniversary, a date officials have long known could draw an attack. Law enforcement officials have been particularly wary after information gleaned from Osama bin Laden's compound in Pakistan indicated that al Qaeda had considered attacking the U.S. on this anniversary and other important American dates. Officials have also been concerned that terrorists would see the anniversary as an opportunity to retaliate against the U.S. for killing bin Laden in a military raid in May.
Officials said that so far they have no reason to believe that there is a direct connection between this new threat and the information found in the compound laying out al Qaeda's aspirational goals.
The FBI and Homeland Security Department issued a joint intelligence bulletin Thursday night to law enforcement around the country urging authorities to maintain increased security and be on the lookout for suspicious activity.
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, officials said.
"There is specific, credible but unconfirmed threat information," the head of the FBI's New York division, Janice Fedarcyk, said. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days."
New York 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, and the city's observance of the attacks will go on as planned.
Bloomberg told CBS' "The Early Show" that, given the extra security precautions, New York City is "the safest place to be"
In Washington, law enforcement officials said they were working 12-hour days indefinitely, and Police Chief Cathy Lanier said unattended cars parked in unusual locations risked being towed.
Law enforcement officials are checking out all of the details included in the threat, Rep. Peter King, R-N.Y., said.
"No need to panic," King said. "They have not been able to confirm it yet."
Intelligence officials had not seen any specific or credible threats regarding an attack around the anniversary before Wednesday.