One official said at least two of them are believed to be connected to al Qaeda in Yemen, and may have a non-Arab appearance and be traveling on Western passports.
The threat was described as “current” but not imminent, said the official.
“They have trained women,” said former White House counterterrorism official Richard Clarke, an ABC News consultant.
Separately, Britain raised its terrorism threat level to “severe,” its second-highest level, days before London hosts major international meetings on how to deal with militancy in Afghanistan and Yemen. Britain’s threat level had been labeled “severe” for several years before being lowered last summer to “substantial.”
American officials say a U.S. air strike on Christmas Eve against suspected al Qaeda training camps is believed to have killed many, but not all, of a group of suicide bombers being trained in Yemen.
The man accused of attempting to explode a bomb on Northwest flight 253, Umar Farouk Abdulmutallab, told FBI agents there were a number of other people who trained with him in Yemen.
“There are others who are still out there who have been trained and who are clean skins — that means people who we do not have a record of, people who may not look like al Qaeda terrorists, who may not be Arabs, and may not be men,” said Clarke.
The alert comes during a week in which American law enforcement officials described an “unusually high” number of people on the no-fly list attempting to board flights to or in the United States.
Six on No-Fly List Stopped in 48 Hours
At least six people on the no-fly list were denied boarding in a 48-hour period between Saturday and Monday this week, according to the officials.
Two of the six were stopped at London’s Heathrow Airport.
On Saturday, an Egyptian man on the no-fly list was stopped from flying on American Airlines flight 113 from London to Miami.
The next day, Sunday, a Saudi Arabian passenger was stopped from boarding United Airlines flight 929 to Chicago. Officials said the man was sent back to Saudi Arabia by the British.
In two other overseas cases involving people on the no-fly list, a man in Nairobi, Kenya was kept off a flight Sunday that would have connected in Amsterdam to Dallas, and a passenger attempting to fly on American Airlines to Los Angeles was stopped in Saint Maarten before he could board a connecting flight to San Juan, Puerto Rico, according to officials.
American officials say there were two additional incidents, in Minneapolis and in Fort Lauderdale, Fla., in which people on the no-fly list were denied boarding, questioned and then allowed to leave the airport without being detained.
“What we don’t know is whether this is because everyone is doing a better job of enforcing the no-fly list, or because the list has been expanded, or because the terrorists are attempting to probe our security,” said ABC News Clarke.
A senior U.S. official said the most likely explanation involved the recent expansion of the no-fly list to include more possible terror suspects.
At the same time, U.S. law enforcement agencies have quietly begun an intense and widespread effort to investigate any American resident who traveled to Yemen in recent months or who was in contact with the radical cleric Anwar Awlaki, who authorities believe serves as an al Qaeda recruiter.
Canadian authorities are also running down leads of any Canadian citizens who have traveled to Yemen, a senior official there said.
Awlaki, an American citizen who lives in Yemen, has been connected by authorities to the accused Northwest bomber, the accused shooter at Fort Hood and several other men convicted of terrorist activity in the United States and Canada.
As part of the additional scrutiny, federal agents are conducting extensive background checks on every passenger who flew to Detroit on the Northwest flight in case one of them might have been sent as a “spotter” on the mission.
Federal agents also tell ABCNews.com they are attempting to identify a man who passengers said helped Abdulmutallab change planes for Detroit when he landed in Amsterdam from Lagos, Nigeria.
Authorities had initially discounted the passenger accounts, but the agents say there is a growing belief the man have played a role to make sure Abdulmutallab “did not get cold feet.”
