A possible Al Qaeda plot to launch an attack during the 10th anniversary weekend of the Sept. 11 terrorist attacks is "looking more and more like a goose chase," a senior U.S. official told Fox News on Saturday.
Federal authorities have been questioning all day the credibility of a tip from a previously reliable source that Al Qaeda had planned to attack Washington or New York, putting though both cities on high alert.
But authorities have not been able to corroborate any of the information from the source.
"The threat is looking less and less credible," the official said, adding that the entire plot as outlined by the source "doesn't seem feasible."
"The time frame doesn't make sense for when these operatives would have been moving into position," the official said. "We are going back to the original source. The president will be briefed on it again in the morning, but people are questioning the credibility of this information at this time. Something is not adding up."
But officials say they won't rest until they review every last detail.
Word that Al Qaeda had ordered the mission reached U.S. officials midweek. A CIA informant who has proved reliable in the past approached intelligence officials overseas to say that three men of Arab descent -- at least two of whom could be U.S. citizens -- had been ordered by newly minted Al Qaeda leader Ayman al Zawahri to mark the 10th anniversary of the 9/11 attacks Sunday by doing harm on U.S. soil.
According to the intelligence, they were to detonate a car bomb in one of the cities. Should that mission prove impossible, the attackers have been told to simply cause as much destruction as they can.
It's still unclear whether any such individuals even exist, according to U.S. officials.
"We don't have a smoking gun yet," Brenda Heck, a top counterterrorism official in the FBI's Washington field office, told Fox News."It is going to take a little bit to completely flush this out. We certainly -- hour by hour -- we are learning more."
Earlier Saturday, the head of the FBI's Washington field office, James McJunkin, said he doesn't expect that there will be any problem "over the anniversary weekend."
If the the tip had not come on the eve of the 9/11 anniversary, the intelligence community likely would not have acted and alerted the public to this degree, the senior official said.
"We couldn't ignore it," the official said. "But something doesn't add up: the routing, the timing of the assets moving into position."
Heck said it's "absolutely possible" authorities will never know whether the alleged plot was in fact real.
In the meantime, extra security was put in place to protect the people in the two cities that took the brunt of the jetliner attacks that killed nearly 3,000 people at the World Trade Center and the Pentagon a decade ago. It was the worst terror assault in the nation's history, and Al Qaeda has long dreamed of striking again to mark the anniversary. But it could be weeks before the intelligence community can say whether this particular threat is real.
The New York Police Department was paying special attention to the thefts of three vans Sunday, scrutinizing them them to eliminate the possibility of their being tied to a larger threat. One van was stolen from a Jersey City facility, while the other two were stolen last week from a company that does work at the World Trade Center site.
Also Sunday, an explosives detection K9 unit alarmed on a cargo pallet as it was being loaded onto a plane at Dulles International Airport. Authorities evacuated several gates as a precaution, but determined there was nothing harmful about the suspicious boxes.
Briefed on the threat Friday morning, President Obama instructed his security team to take "all necessary precautions," the White House said. Obama still planned to travel to New York on Sunday to mark the 10th anniversary with stops that day at the Pentagon and Shanksville, Pa.
Heck, the FBI counterterrorism official, said the government's response to the latest threat "has been a little different" than at other times.
"We have been very open with the public on this," she said. "I think there will be some debate about that after we get through this weekend. [But] I think there's a very positive side to letting the public know a little bit more about what we are doing behind the scenes."
In particular, she said, by letting the public know about a threat quickly, "They can help us with what's going on out in the public areas so that we can respond if something is suspicious."
In fact, Washington Police Chief Cathy Lanier said suspicious reporting has surged by as much as 30 percent, a change that she called "very reassuring."
Fox News' Mike Levine, Catherine Herridge, Jennifer Griffin and The Associated Press contributed to this report.