Search crews will not be able to retrieve the black box at the Pentagon, which could contain information about the last minutes of the hijacked commercial jetliner, until they are able to enter the collapsed area of the Pentagon, where the plane's fuselage rests.

They were to begin moving into the collapsed area sometime Thursday night, said Arlington County Fire Capt. Scott McKay.

While there have been no arrests, Ashcroft said, authorities have interviewed many people in connection with the hijacking of four airliners and the attacks on the World Trade Center in New York and the Pentagon.

A total of 18 hijackers were on the planes, Ashcroft said. There were five on each of two planes and four each on the other two. All have been identified, officials said.

He said he had was heartened by the public's interest in tracking down those responsible.

"The FBI is working thousands and thousands of leads," he said.

Ashcroft said the FBI's 800-number hot line had received 2,055 calls. In addition, its Web site had received more than 22,700 tips, he said.

"Some of these leads have been helpful to the investigation," Ashcroft said.

He noted that authorities were still searching for the flight-data and cockpit voice recorders of all four planes that crashed -- two in New York, one at the Pentagon and the other in southwestern Pennsylvania.

Mueller said all 18 hijackers on the four planes were ticketed passengers.

Earlier, the Justice Department said that at least one hijacker on each plane was trained at a U.S. flight school and that well over 50 people may have been involved in the hijackers' well-financed operation.

A number of people that could be involved in the plot were detained overnight for having false identifications, Justice Department spokeswoman Mindy Tucker said earlier Thursday. She declined to say how many were detained or where they are being held.

Officials are close to releasing the names and possibly the country of origin of the hijackers. Nearly all have been identified, Tucker said.

The FBI's massive investigation stretches from the Canadian border to Florida, where some of the participants learned how to fly commercial planes before the attacks. Tucker said flight schools in more than one state were involved in the training of the hijackers, several of whom had pilots' licenses.

Multiple cells of terrorist groups participated in the operation and the hijackers had possible ties to countries that included Saudi Arabia and Egypt, said law enforcement officials, speaking on condition of anonymity.

Officials said authorities were gathering evidence that the terrorist cells may have had prior involvement in earlier plots against the United States, and may have been involved with Saudi exile Osama bin Laden. That includes the USS Cole bombing in Yemen and the foiled attack on U.S. soil during the millennium celebrations.

In Florida Thursday, FBI agents were interviewing three Saudi Arabian flight engineers who are taking classes at Flightsafety International's flight school in Vero Beach, Fla., 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 spokesman for Saudi Arabian Airlines, said many of the airline's pilots came to the United States for flight training.

About 40 of the people involved in the attacks have been accounted for, including those killed in the suicide attacks, but 10 remain at large, the Los Angeles Times reported, citing an unidentified source with knowledge of the investigation.

Some of those involved in the plot left suicide notes, but they are not believed to have been the hijackers, a government source told The Associated Press. It's unclear whether those who left the notes actually killed themselves.

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