FBI officials said yesterday that some of the 19 terrorists who carried out last week's assault on New York and Washington may have stolen the identities of other people, and their real names may remain unknown.

Saudi government officials also said yesterday that they have determined that at least two of the terrorists used the names of living, law-abiding Saudi citizens. Other hijackers may have faked their identities as well, they said..

FBI Director Robert S. Mueller III said Friday that the bureau had "a fairly high level of confidence" that the hijacker names released by the FBI were not aliases. But one senior official said that "there may be some question with regard to the identity of at least some of them."

The uncertainty highlights how difficult it may be to ever identify some of the hijackers who participated in the deadliest act of violence on American soil. Most of the hijackers' bodies were obliterated in the fiery crashes.

"This operation had tremendous security, and using false names would have been part of it," said John Martin, retired chief of the Justice Department's internal security section. "The hijackers themselves may not have known the others' true names."

The identity problem adds to the steep challenges facing the FBI and other federal investigators as they race to hunt down suspected conspirators in the Sept. 11 attacks on the World Trade Center and the Pentagon, which are believed to have killed nearly 6,000 people.

In other developments yesterday:

* Investigators now believe accomplices of the hijackers may have tried to confuse air traffic controllers by making a series of false bomb threats against airliners on the morning of Sept. 11, sources close to the case said.

* New information from law enforcement officials indicates that at least 44 of the people the FBI has sought for questioning in the probe are trained pilots. One of them, a man identified as Ayub Ali Khan, was arrested as a material witness in the case after being detained in Texas. He was carrying box-cutter knives like those believed to have been used in some of the four hijackings.

Other potential suspects include a native of Yemen who investigators believe may also have been involved in the bombing of the USS Cole, and two men -- both in Jordanian custody -- who were arrested in connection with a bomb plot related to the millennium celebrations. U.S. intelligence officials have linked both of those events to Saudi militant Osama bin Laden.

* Sen. Bob Graham (D-Fla.), chairman of the Senate Select Committee on Intelligence, repeated warnings of other, unspecified threats, but officials said none had been confirmed.

Some reports have cited Sept. 22 as a potential date for additional attacks in part because alleged associates of the hijackers purchased airline tickets for flights that day from San Antonio to San Diego. But Justice Department spokeswoman Mindy Tucker said, "There is no credible evidence of any threat for Sept. 22."

By yesterday evening, officials said they had detained 115 people in connection with the probe. Officials continue to work through a list of about 190 others sought for questioning.

Gaafar Allagany, chief of the Saudi Embassy's information office in Washington, said yesterday that the Saudis are convinced that most, if not all, of the suspected hijackers named last week by the FBI used stolen identities. In two cases that the Saudis say they have confirmed, passports were stolen several years ago.

One of the terrorists who the FBI said died aboard the American Airlines flight that crashed into the Pentagon used the name of Salem Alhazmi.

"The Hazmi we have is 26 years old and has never been to the United States," Allagany said. He said the man, whose picture has been published as that of the dead terrorist, works at a government-owned petrochemical complex in the Saudi city of Yanbu.

"He has shown authorities there that he has not left Saudi Arabia in two years," Allagany said.

Alhazmi has told reporters in Saudi Arabia that his passport was stolen by a pickpocket on a trip to Cairo three years ago. His picture was published yesterday in a Saudi newspaper, Al-Sharq Al-Awsat, alongside that of the man the Saudis believe to be the dead terrorist, Badr Alhazmi.

Another Saudi, Abdulaziz Alomari, had his passport and other papers stolen in 1996 when he was a student in Denver and reported the theft to police there, Allagany said.

A hijacker identified by the FBI as Abdulaziz Alomari was aboard the American Airlines flight from Boston that crashed into the North Tower of the World Trade Center. He was said to have used different birth dates and was believed to have been a pilot.

Allagany said the real Abdulaziz Alomari is an electrical engineer in Saudi Arabia.

The names identifying the hijackers were obtained through searches of passenger manifests combined with information from financial records and other sources, officials said. Investigators have concluded that at least some of the names could have been stolen from other people.

Investigators have found that the hijackers and some of their suspected accomplices have freely swapped names, credit cards, e-mail accounts and other identifying information.

For example, Badr Mohammed H. Hazmi, a San Antonio radiologist under arrest as a material witness, has used the alias of Khalid Al-Midhar, who is listed by the FBI as a hijacker on the flight that struck the Pentagon, according to documents distributed to law enforcement agencies.

The father of another alleged hijacker, Mohamed Atta, told reporters in Cairo earlier this week that he did not believe his son was a terrorist and said that his documents may have been stolen. He also said on Tuesday that he had spoken to his son "four or five days ago . . . about 48 hours after" the attacks, according to the Associated Press.

New information from law enforcement sources and documents suggests previously unknown connections among the suspected hijackers and men believed to have been their accomplices.

For example, officials are seeking several men who have the same last name as Atta, a suspected hijacker of American Airlines Flight 11. One is Asem Atta, a Pakistani national and a former resident of Kuwait who is being sought for questioning in connection with the case. He lived in a downtown Houston apartment building until a few weeks before the attacks, when he disappeared, a neighbor said.

Dozens of those sought for questioning were trained as pilots. Khan, one of the two men arrested with box-cutters on an Amtrak train bound for San Antonio, was a commercially certified pilot, law enforcement documents show. Khan and Mohammed Jaweed Azmath, both from India, boarded TWA Flight 679 on Sept. 11, which left Newark at 6:10 a.m. The plane was grounded in St. Louis after the terrorist attacks, and Azmath and Khan boarded the train.

In Detroit, federal agents found two former airport catering workers and one other man in possession of fraudulent documents and a day planner that included notations in Arabic relating to the "American base in Turkey," the "American Foreign Minister" and "Alia Airport" in Jordan, according to court records.

Authorities arrested the three men while looking for another man, Nabil Al-Marabh, who is believed by the FBI to be an associate of Raed Hijazi. Hijazi is in custody in Jordan in connection with the millennium bomb plots.

Authorities are also examining whether there are links between the attacks and two New Jersey men who were arrested in June for allegedly acting as go-betweens for a foreign buyer interested in Stinger missiles, night-vision goggles and nuclear weapons components.

The attorney for Diaa Mohsen, 57, of Jersey City, said federal agents interviewed his client late last week in the federal detention center in Miami where he is being held. Mohammed Rajaa Malik, 52, of Watchung, N.J., has not been contacted, his attorney said.

An Orlando man arrested Sunday as a material witness appeared Wednesday in U.S. District Court for the Middle District of Florida. After a 15-minute detention hearing, U.S. Magistrate David Baker granted a government motion to close the proceedings and seal the court record in the case because it "is related to a grand jury proceeding in another district," possibly one in White Plains, N.Y.

In Dania Beach, Fla., the owner of U.S. 1 Fitness said that one of the men suspected of hijacking the United Airlines flight that crashed near Pittsburgh spent several months learning "close quarter grappling" and how to use knives and sticks in combat.

Bert Rodriguez said he provided regular training for a man who gave his name as Ziad Jarrah and paid with cash. The center's records show he was a member of the gym from May 7 to Sept. 7.

Farther up the coast, five suspected terrorists' memberships at World Gyms in Delray Beach and Boynton Beach have attracted the FBI's attention. The FBI has reviewed records showing that two of the suspects paid using personal Visa cards, the gyms' owner said.

Staff writers Justin Blum, Michael A. Fletcher, Amy Goldstein, Charles Lane, Serge F. Kovaleski, Allan Lengel, Bill Miller and Lena H. Sun contributed to this report.

Attorney General John D. Ashcroft inspected damage at the Pentagon as the possibility that identities were stolen toughened the FBI's task.