WASHINGTON - Federal authorities investigating Tuesday's devastating attacks are focusing on multiple separate terrorist groups, some tied to Osama bin Laden, law enforcement officials said Wednesday.

Members of one organization may have entered the United States through Canada, authorities said.

The FBI has identified most of the hijackers who commandeered and crashed four airliners, and suspected accomplices, officials said. At least five people were detained; others were interviewed in the hunt for accomplices. No one has been arrested.

A number of the suspected hijackers were trained as pilots in the United States. Their names were not immediately disclosed by authorities.

Intelligence officials are pursuing "numerous credible leads," Attorney General John Ashcroft said.

"The Department of Justice has undertaken perhaps the most massive and intensive investigation ever conducted in this country," he said.

FBI agents obtained information from Internet providers, conducted searches, and questioned people in Florida and Massachusetts. Early evidence, including communications among bin Laden supporters, indicated the attacks were tied to the wealthy Arab and accused terrorist.

Ashcroft said authorities were conducting interviews and reviewing airline manifests, rental car records and pay phone records. He said between three and six hijackers, armed with knives and box cutters, seized control of the four commercial jets. Two hit New York's World Trade Center, a third smashed into the Pentagon and a fourth crashed in Pennsylvania.

For some of the suspected accomplices, "we have information as to involvement with individual terrorist groups," FBI Director Robert Mueller said. He declined to say which groups or whether they were connected to bin Laden.

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

"This could have been the result of several terrorist kingpins working together. We're investigating that possibility," one law enforcement official speaking on condition of anonymity told The Associated Press.

Mueller said agents have followed leads that the hijackers or their associates had been in Florida, Boston and Providence, R.I. He said authorities are "attempting to recreate the travels" of the suspected attackers.

Some 4,000 special agents and 3,000 support personnel are assisting in the investigation, and 400 FBI laboratory specialists are at the crime scenes in New York, Washington and Pennsylvania.

Evidence has been collected at the Pentagon and Pennsylvania site, but investigators have not yet been able to start work at the World Trade Center, where the search for survivors continued.

The officials, who spoke on condition of anonymity, said they were investigating whether one group of hijackers crossed the Canadian border at a checkpoint and made their way to Boston, where an American Airlines flight was hijacked and flown into the World Trade Center.

The officials confirmed a car believed to belong to the hijackers was confiscated in Boston and contained an Arabic language flight manual.

Abu Dhabi Television in the United Arab Emirates reported that two men with Saudi Arabian passports and international drivers licenses issued in the UAE were linked to the Mitsubishi sedan found at the Boston airport.

Law enforcement officials said that the FBI on Wednesday afternoon searched two hotel rooms in the Boston area believed to have been used by the hijackers. The officials found information linked to a name on the manifest of one of the hijacked flights. They declined to identify the man.

A Venice, Fla., man said FBI agents told him that two men who stayed in his home while training at a local flight school were the hijackers. Charlie Voss said the agents identified the men as Mohamed Atta and one known as Marwan.

The FBI in Miami issued a national bulletin for law enforcement agencies to look out for two cars. Records with the Florida Division of Motor Vehicles show that one of the vehicles the FBI was pursuing - a 1989 red Pontiac - was registered to Atta.

The FBI has already received more than 700 tips from a special Web site seeking information on the attacks.

Agents served search warrants on major Internet service providers in order to get information about an e-mail address that may be connected to the attacks.