Associated Press WriterWASHINGTON (AP) -- At least one hijacker on each of the four planes in Tuesday's terrorist attacks was trained at a U.S. flight school, authorities say, pressing an investigation Attorney General John Ashcroft called the largest in history.
Overall, 50 people may have been involved in the hijackers' well-financed operation.
"We are pursuing thousands of credible leads," Ashcroft said Thursday. "We're making some progress."
Ashcroft, on the morning talk shows, said authorities were deploying hundreds of U.S. marshals and other agents to airports and airplanes Thursday to increase security with the gradual resumption of commercial flights.
He was not specific on the agents' duties, nor would he say whether arrests of hijackers' accomplices were imminent. But he noted: "There's a sense of urgency that's understood by all of us."
Two of the hijacked planes destroyed New York's World Trade Center, one plane heavily damaged the Pentagon and a fourth plane crashed in southwestern Pennsylvania.
The FBI's investigation stretches from the Canadian border to Florida, where some of the participants learned how to fly commercial planes before the attacks. Justice Department spokeswoman Mindy Tucker said flight schools in more than one state were involved in the training of the hijackers, several of whom had pilots' licenses.
Hijackers used both cash and credit cards to "purchase tickets, hotel rooms and other things," she said Wednesday.
Ashcroft said the search for accomplices was complicated by the fact that the hijackers were dead. But he said a trail has been left for the law to follow. The FBI alone has deployed some 4,000 special agents and 3,000 support personnel.
"We need to work this as expeditiously as possible," he said. "That's why we've launched the largest investigation in the history of the United States."
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.
The identities of more than a dozen of the men who hijacked the four planes with knives and threats of bombs have been ascertained, the officials said.
For some suspected accomplices, "we have information as to involvement with individual terrorist groups," FBI Director Robert Mueller said.
Ashcroft said 12 to 24 hijackers commandeered the four planes, and a government official said another two dozen or so are believed to have assisted them. About 40 of the men have been accounted for, including those killed in the suicide attacks, but 10 remain at large, the Los Angeles Times reported Wednesday night on its Web site, citing an unidentified source with knowledge of the investigation.
The Times reported at least one of the suspects receiving advanced flight training in Florida was a commercial pilot from Saudi Arabia.
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.
At least one hijacker on each of the four planes was trained at a U.S. flight school, Justice spokeswoman Tucker said. The Times said authorities believe 27 suspected terrorists received pilot training.
Authorities detained at least a half dozen people in Massachusetts and Florida on unrelated local warrants and immigration charges and were questioning them about possible ties to the hijackers.
Search warrants were executed in Florida, New Jersey and Massachusetts. Sealed warrants went out in several other states, officials said.
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 involved in the attacks. Charlie Voss, a former employee at Huffman Aviation in Venice said the FBI told him one of men was named Mohamed Atta. A student at Huffman Aviation identified the second man as Marwan Alshehhi.
Citing federal authorities, The Miami Herald reported Thursday that Atta was one of four suspects who died on American Airlines Flight 11, the first jetliner to crash into the World Trade Center.
"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 AP.
Sen. Charles Grassley of Iowa, the top Republican on the Senate Finance Committee, said the briefing he received Wednesday from law enforcement left him with the same impression.
"Most of it today points to Osama bin Laden, but the speculation at the end of the road is that he and his network were very much involved with Hezbollah, Fatah and other" terrorist organizations, Grassley said.
The senator said authorities told him all the hijackers were of Middle Eastern descent and that there was "a tremendous amount of ground support for each hijacker."
Attorney General John Ashcroft said numerous promising leads were being followed up. "The Department of Justice has undertaken perhaps the most massive and intensive investigation ever conducted in this country," he said.
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 two jetliners were hijacked and flown into the World Trade Center.
Two suspects flew from the Portland International Jetport in Maine to Boston, where they boarded the deadly flights, Maine Gov. Angus King said. The two men apparently were using New Jersey driver's licenses and left behind a rental car with Massachusetts plates that was impounded and hauled to the Maine State Police crime lab.