WASHINGTON — FBI officials said yesterday some of the 19 terrorists who carried out last week's assault on New York and near Washington may have stolen the identities of other people and their real names may never be known.

Saudi government officials also said yesterday they think at least two of the terrorists used the names of living, law-abiding Saudi citizens, and other hijackers may have faked their identities, too.

FBI Director Robert Mueller III had said Friday the bureau had "a fairly high level of confidence" the hijacker names released by the FBI were not aliases. But one senior official said yesterday "there may be some question with the 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 in the deadliest act of violence on U.S. 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 federal investigators as they race to hunt down suspected conspirators in the Sept. 11 attack on the World Trade Center and Pentagon, which may have killed nearly 6,000 people.

In other developments yesterday:

• Portland, Maine, police yesterday released a photo from a surveillance tape showing two suspected terrorists. The image showed men believed to be Mohamed Atta and Abdulaziz Alomari before boarding a commuter flight to Boston, where the men switched to American Airlines Flight 11, which

was hijacked and crashed into the World Trade Center.

• Investigators now think accomplices may have attempted 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 at least 44 of the people the FBI has sought for questioning are trained pilots. One of them, a man identified as Ayub Ali Khan, was arrested as a material witness after being detained in Texas. He was carrying box-cutter knives like those thought to have been used in some of the hijackings.

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

• The Treasury Department said yesterday that new law-enforcement teams in Chicago and San Francisco will join an existing one in Los Angeles in investigating and prosecuting money laundering by suspected terrorists. The scheme may have been funded with money from Islamic charities and front groups and wealthy Islamic businessmen, terrorism experts say.

• Several suspected hijackers in last week's terrorist attacks stayed at motels in suburban Washington just before the attack, some working out in a gym and eating at a pizzeria, residents and employees told the FBI.

Business owners said that in recent days FBI agents questioned them and showed photos of suspects. FBI spokesman Peter Gulotta would not comment.

• Sen. Bob Graham, D-Fla., chairman of the Senate Select Committee on Intelligence, repeated his warning that "there is evidence that Tuesday's attack was the first phase of a multiphase series of terrorist assaults against the United States, all under one umbrella plan."

However, Justice Department spokeswoman Mindy Tucker said, "There is no credible evidence of any threat for Sept. 22." The date has received wide attention, in part, because alleged associates of the hijackers purchased airline tickets for flights that day from San Antonio to San Diego.

By yesterday evening, officials said they had detained 115 people in connection with the probe on suspected immigration violations. 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 the Saudis are convinced 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 so far, passports were stolen several years ago.

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

"The Alhazmi 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. "He has said that he is willing to come to the United States if anyone wants to see him."

Alhazmi has told reporters in Saudi Arabia that his passport was stolen 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 think is the dead terrorist, Badr Alhazmi.

Another Saudi, Abdulaziz Alomari, said his passport and other papers were stolen in 1996 when he was a student in Denver, and he reported the theft to police there, Allagany said. Alomari obtained a special pass from the Saudi Embassy to return home, where he paid a small fine before obtaining a new passport.

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 thought to have been a pilot.

Allagany said the real Abdulaziz Alomari is an electrical engineer now working for a company in Saudi Arabia. He discovered his Denver apartment had been looted five years ago, Allagany said.

Information from The Associated Press is included in this report.