Thousands of FBI agents fanned out across the country yesterday in the hunt for accomplices to the terrorist attacks on New York and Washington, executing search warrants from Boston to Florida and apprehending four individuals before releasing at least three of them.

Using airplane manifests, passport records and other sources, U.S. officials believe they have successfully identified most of the suicidal hijackers, who numbered at least 12 and possibly as many as 24 individuals and included several pilots who received flight training in the United States, according to Attorney General John D. Ashcroft, FBI Director Robert S. Mueller III and other officials.

"The four planes were hijacked by between three and six individuals per plane, using knives and box cutters and, in some cases, making bomb threats," Ashcroft said.

At least one hijacker on each plane received flight training in the United States and several received pilot's licenses, Justice Department spokeswoman Mindy Tucker said last night. She said flight schools in several states were part of the FBI's investigation.

The revelation that some of the pilots who carried out Tuesday's hijackings were schooled in the United States underscores the astonishing degree of organization and planning undertaken by the terrorists, who had the ability to recruit and train numerous pilots in preparation for the precision attacks.

The complexity of the case is further underscored by the size of the Justice Department probe, which Ashcroft called "perhaps the most massive and intensive investigation ever conducted in America." Mueller said 4,000 special agents and 3,000 support workers, or about 25 percent of FBI employees, have been assigned to the case, including more than 400 FBI crime lab experts who have been sent to the crash sites.

As part of the mobilization yesterday, the FBI detained a man for questioning and served search warrants on at least four locations in Florida, while heavily armed federal agents detained three people for several hours after law enforcement officials traced credit card receipts from a car rental agency to a guest at a hotel in downtown Boston. Law enforcement officers also searched a hotel in Newton, Mass., outside Boston.

Ashcroft, who briefed members of Congress on the investigation, said investigators were obtaining passenger manifests, rental-car receipts, telephone logs and videotape from parking garages and pay phones, particularly in the places where the hijacked flights took off.

Canadian security officials are also investigating whether as many as five participants in the deadly attack used Canada as a staging ground, slipping across the border from Nova Scotia shortly before the hijackings. Two of the hijacked planes took off from Logan International Airport in Boston; the others were from Newark International Airport and Washington Dulles International Airport.

Those detained in the United States as of late afternoon yesterday were being held on immigration violations but had not been arrested or charged with any crime, according to Mueller and others.

Ashcroft also said the "government has credible evidence that the White House and Air Force One were targets" of the terrorists, who succeeded in flying two hijacked airliners into the 110-story World Trade Center towers in New York and a third into the Pentagon in Washington. A fourth crashed in southern Pennsylvania after passengers apparently mounted a revolt, according to accounts of cell phone conversations.

"We will leave no stone unturned until we have determined who was responsible for these attacks on our freedom," said Mueller, who is in his second week on the job.

Mueller said some of the suspected hijackers and their accomplices had ties to several terrorist groups, but he declined to provide details.

But other U.S. officials said that several of the groups are known to have ties to Saudi fugitive Osama bin Laden, an extremist Islamic militant whose organization, al Qaeda, has been linked to numerous terrorist bombings, including a previous attack on the World Trade Center in 1993 and the bombings of U.S. embassies in Kenya and Tanzania in 1998.

After a briefing yesterday by intelligence officials, Sen. Kay Bailey Hutchison (R-Tex.), a member of the Armed Services Committee, said authorities have discerned a "pattern of travel" by about 15 individuals, including five who have been found to be carrying passports from two Middle Eastern countries. She did not identify the countries.

Hutchison said the 15 were found to have gone to one or more other countries before entering the United States. Canada was not specifically mentioned, she added.

Asked about bin Laden's possible connection to the attacks, she said, "He is certainly high on the list . . . but he is not the only one they're looking at."

In Florida, search warrants were served on homes in Coral Springs, located northwest of Fort Lauderdale, on a home in Vero Beach along Florida's east coast, and on businesses in Hollywood and a home in Sarasota County on the west coast, according to police and witness reports.

Officials said many of yesterday's searches and detentions were prompted by a review of the passenger manifests of the four hijacked planes.

In Vero Beach, FBI agents searched the home of a Saudi Arabian pilot who had received flight engineering training along with at least one other Saudi at the local Flight Safety International training school, according to the home's property owner. Among the items seized by the FBI was a "hazardous materials manual," according to a copy of an FBI document left on the home's kitchen table.

Landlord Paul Stimeling said the man, Adnan Bukhari, helped a second Saudi pilot rent the house next door. The second pilot and his family, including his wife and as many as five children, moved out over the weekend and have not been seen since, Stimeling said.

Both of the men said they were Saudi nationals and flew for Saudi Airlines, Stimeling said. FBI agents swarmed the neighborhood yesterday morning and spent several hours searching the homes, witnesses said.

Also in Vero Beach, FBI agents detained an unidentified man in connection with the case.

The Immigration and Naturalization Service is investigating whether two of the hijacking suspects may have come into the country with "M" visas, a rarely issued nonimmigrant visa that allowed them to attend the Florida flight school, according to a former senior INS official who has talked to investigators. INS officials are also concerned that one or more of the suspects may have entered the United States while seeking asylum.

In Coral Springs on Tuesday night and again yesterday morning, FBI agents searched an apartment rented to Mohammed Atta, according to residents of the Tara Gardens complex. The FBI said a vehicle registered to Atta was connected to the case. FBI agents also questioned a 17-year-old girl who is the friend of a resident of the complex who had seen Atta twice before in the lobby and they showed her a photograph of him.

Atta listed this address when he got a Florida driver's license in May. According to driver's license records, he previously had an Egyptian driver's license.

On Tuesday, FBI agents visited a Shuckums restaurant in Hollywood with photographs of the man believed to be Atta and another heavyset man, according to Tony Amos, the restaurant's manager.

On Friday evening, Amos said Atta and the other man came into the Shuckums. The pair had been in the restaurant several times before, Amos said. This time, they were there from about 4:30 p.m. until about 7 p.m.

"My bartender said, 'These guys are giving me a hard time,' " Amos recalled yesterday. "I thought, 'Okay, here we go.' I was talking to the big guy, going back and forth. I said, 'Is there a money issue? Do you have enough money to pay the bill?' The guy arrogantly replied, 'I'm an airline pilot. Of course, I have the money.' "

Amos said they paid the $48 bill in cash.

In Boston's Copley Square area, local police and agents from the FBI and the Bureau of Alcohol, Firearms and Tobacco descended on the Westin Copley Hotel with battering rams and shields, detaining three men who were tracked using a credit card receipt from a car rental, officials said.

Several of the hijackers may have entered the United States shortly before the attacks by taking a ferry from Nova Scotia to Portland, Maine, according to several sources.

Two of the terrorists flew to Boston from Portland, where law enforcement officials recovered their rental car, according to Maine Gov. Angus King. Another source close to the investigation said another group of at least two hijackers drove from Maine to Boston after making a ferry trip from Nova Scotia into the United States.

Bin Laden, 44, who comes from a wealthy, extended Saudi family, has strong family ties and a group of supporters in Boston, where the two hijacked airliners that demolished the World Trade Center took off. One of bin Laden's brothers set up scholarship funds at Harvard, while another relative owns six condominiums in an expensive complex in the Charlestown section of Boston.

Two bin Laden associates once worked as Boston cab drivers, including one who was jailed in Jordan on charges of plotting to blow up a hotel full of Americans and Israelis.

U.S. and Canadian officials believe that as many as five of the suspects involved in hijacking one of the planes from Boston may have entered the United States after boarding a ferry in Yarmouth, Nova Scotia.

"The security people are looking to find out who are these people and where they were coming from," Canada's Prime Minister Jean Chretien said at a news conference in Ottawa. "There is some indication they used Canada to get there. But we don't know. There are inquiries being done in the United States and in Canada. The police are doing their job on both sides."

Providence, R.I., Mayor Vincent A. Cianci Jr. said local police stopped an Amtrak train yesterday and questioned several men at the request of Boston police. One man was detained and arrested for possession of a knife, Cianci said. But Cianci said the man is not believed to be connected to Tuesday's attacks.

FBI DIRECTOR ROBERT S. MUELLER IIIBoston police prepare to enter the Westin Copley Hotel, where three people were detained and then released. Credit card receipts were traced to a guest there.A crowd gathered outside Boston's Westin Copley Hotel after police in special operations gear entered.