sm officials said Friday that authorities have a general sense of who is behind the plot and who may be tasked to carry it out, but they don't have exact identities yet. They said there is at least one U.S. citizen among the three people. The intelligence suggested that al-Qaida planned to car bomb one of the two cities that were hit 10 years ago, in coordinated attacks that killed nearly 3,000 people. 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. U.S. officials 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. The nation's terror alert level has not changed, although raising it was under consideration Thursday night. 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. The officials described the threat to The Associated Press on condition of anonymity because they were not authorized to publicly discuss sensitive security matters. 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-Qaida 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-Qaida'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 speci