WASHINGTON -- U.S. investigators pressed Thursday to identify terrorist collaborators who may still be in a position to strike more Americans, and agents located critical ''black boxes'' from two of Tuesday's hijacked planes.
Four U.S. officials, speaking on condition of anonymity, told The Associated Press that authorities are investigating the possibility that some terrorists involved with Tuesday's plots are still at large.
Five men who tried to board a plane Thursday in New York were being questioned, officials said. One of the men had a false pilot's identification. The five were identified as the same men who had tried to board a plane around the time of Tuesday's hijackings, but were turned away.
The FBI sent the airline industry a list of 52 people wanted for questioning. Airlines were asked to alert agents if any of the individuals were spotted.
The FBI searched the country and abroad for possible suspects who had recent flight training, ties to the hijackers or their backers, or attempted to enter the United States recently, said these 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. He cited concerns terrorists may strike in a different manner now that airport security has been beefed up.
Signs of fear were everywhere. The U.S. Capitol was evacuated for a suspicious package and New York's airports were temporarily closed to incoming flights. One man was arrested in New York with a fake pilot's identification. 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 Naturaliza-tion Service, a Justice Department official said.
The department had previously said people were detained, including at least a half dozen in Massachusetts and Florida, because of immigration problems. But it wasn't until late Thursday that officials revealed that those people had been arrested.
The INS has 48 hours after arrest to charge someone violating immigration rules. Some of those detained could be charged Friday, said the official, speaking on condition of anonymity.
No one has yet been charged in Tuesday's attacks.
In Minnesota, the possibility emerged that the FBI knew before Tuesday's attack of at least one Arab man seeking the type of flight training the hijackers received.
U.S. officials confirmed that a few weeks ago the FBI detained an Arab man in Minnesota when he tried to seek flight simulator training for a large jetliner. Those who hijacked the four airliners received similar training.
Officials said the FBI had no reason to charge him at the time and instead began deportation proceedings. Those proceedings were ongoing when the attacks took place Tuesday, and he was re-detained.
He was not cooperating with the FBI.
Investigators recovered a black box flight recorder from the hijacked plane that went down in Pennsylvania, and picked up a signal from the recorder in the jet that slammed into the Pentagon.
The recorders could contain information about the last minutes of the hijacked commercial jetliners.
FBI Special Agent Bill Crowley said the recorder in Pennsylvania was found at about 4:20 p.m. EDT in the 8-foot crater caused by the crash. Crowley said the recorder would be analyzed by the National Transportation Safety Board.
''We're hoping it will have some information pertinent to what happened on the plane,'' 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.
Overseas, 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.
Acting on a tip from the FBI, police in Hamburg detained one man and were seeking another. The police did not say how the detainee might have been linked to the attacks.
An FBI official was 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.
Attorney General John Ashcroft said a total of 18 hijackers took over the four planes. The Justice Department had planned to release the hijackers' names and photos, but pulled back late Thursday.
All were ticketed passengers but some may have used aliases, officials said.
Elsewhere, authorities were looking for a Muslim cleric who previously was questioned by prosecutors in the 1990s embassy bombings case linked to bin Laden.
The cleric, Moataz Al-Hallak, left the Northeast on Monday, the day before the attacks, and traveled to Texas, according to authorities and his lawyer.
Al-Hallak's lawyer, Stanley Cohen, said FBI agents want to question his client about whether he told people about the attacks before they occurred.
''I asked Moataz about it, and he was shocked and just laughed. It's preposterous,'' Cohen said.
Al-Hallak appeared before a federal grand jury in New York in the case of the U.S. Embassy bombings in Africa linked to bin Laden. He was never charged with wrongdoing.
A number of people that could be involved in the plot were detained overnight for having false identification, Justice Department spokesperson Mindy Tucker said.
One focus of the FBI is on flight schools in Florida and Minnesota that trained some of the men apparently involved in the hijackings. The owner of a Minnesota flight school said FBI agents had contacted him asking about specific people.
In Florida on Thursday, FBI agents were interviewing three Saudi Arabian flight engineers who are taking classes at Flightsafety International's school in Vero Beach, company spokesman Roger Ritchie said. He declined to name the engineers.
The school does not have simulators for Boeing 767 and 757 aircraft such as the ones involved in Tuesday's attacks, Ritchie said.
Thomas Quinn, a New York-based spokesperson for Saudi Arabian Airlines, said many of the airline's pilots came to the United States for flight training.
The FBI questioned a Fort Smith, Ark., couple after telling police agencies across the state Wednesday that the woman was ''possibly related to the New York City terrorist attack,'' state police spokesperson Kim Fontaine said. The husband was being held Thursday for federal immigration officials and the woman was taken away by agents and her whereabouts were not released, the Southwest Times Record newspaper at Fort Smith reported.
Associated Press writer Karen Gullo contributed to this report.
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