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.
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 data ``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.
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 was found at about 2020 GMT in the 8-foot (2.4-meter) 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.''
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.