WASHINGTON -- At full throttle, U.S. intelligence and law enforcement officials are trying to identify collaborators in this week's terrorist attacks to ensure they don't strike again.
The effort yielded a dramatic result late Thursday in New York, where airports were abruptly shut and authorities apprehended at least five men being sought for questioning in connection with Tuesday's attacks by suicide hijackers.
Two groups of passengers of Middle Eastern descent who were detained at two New York airports were later determined by the FBI to have no connection to the terrorist attacks, Sen. Joseph Biden, D-Del., chairman of the Senate Foreign Relations Committee said Friday.
The incident had prompted authorities to close the region's three major airports, a U.S. official said. But Biden told CNN that suspicions about a possible link between the as many as 10 men and the attacks turned about to be "totally, totally coincidental."
Struggling to get back to normal operations, U.S. airlines earlier had received a list from the FBI of 52 people, most of Middle Eastern descent, whom authorities wanted detained if they appeared at airports. Several people across the country were being questioned or held on immigration charges.
"It's pretty clear that there were probably others involved in these endeavors. And it's our interest to track those individuals down," Attorney General John Ashcroft said.
Justice officials said they believed 18 men were directly involved in the hijackings with possible ground support from dozens more. Those who made it on the ill-fated planes were ticketed passengers but some apparently used aliases, officials said.
The FBI searched worldwide for possible suspects who had recent flight training, ties to the hijackers or their backers, or attempted to enter the United States recently, according to four officials who spoke only on condition of anonymity.
Agents have been examining manifests of flights that were not hijacked on Tuesday to find matches with people who fit this profile, the officials said.
The concerns are also being driven by fresh intelligence suggesting a continuing threat, the officials added. The information "suggests we haven't seen the end of this current threat," one U.S. official said.
Signs of concern were evident. The Capitol was evacuated for a suspicious package Thursday, and a security ring around the White House was widened.
A number of people questioned in connection with the plot have been arrested for immigration violations and were in the custody of the Immigration and Naturalization Service, the Justice Department said.
Early Friday, investigators recovered the voice and data recorders from the jet that slammed into the Pentagon. On Thursday, searchers found the flight data recorder from the hijacked plane that went down in Pennsylvania.
The recorders could contain information about the last minutes of the hijacked commercial jetliners.
"We're hoping it will have some information pertinent to what happened on the plane," FBI Special Agent Bill Crowley said. "This development is going to help a lot."
The FBI has a transcript of communications between the pilots and air traffic controllers for a portion of the flight that crashed in Pennsylvania, but has not yet released it, officials said.
Elsewhere, U.S. and Philippine authorities searched a Manila hotel in connection with the investigation. Philippine officials also questioned a Saudi Airlines pilot and refused entry to nine Malaysian men suspected of having undergone terrorist training.
German authorities said three of the terrorists who died in the suicide attacks were part of a group of Islamic extremists in Hamburg who have been planning attacks on the United States. Hamburg investigators said two of the terrorists were Mohamed Atta, 33, and Marwan Alshehhi, 23, whose training at a Florida flight school has been the focus of intense FBI interest this week. The German investigators said the two were from the United Arab Emirates.
An FBI official headed for the Azores Islands to interview two Iranians detained a week ago after they tried to travel to Canada with fake passports, authorities said.
Mexican Defense Secretary Gen. Rafael Macedo said officials are searching the country for at least nine people who may have helped plan the attacks.
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