The FBI is investigating whether hijackers were on board two planes which failed to take off in the United States on the same morning as four others crashed in the terrorist attacks.
Federal investigators believe terrorists could have been on an American Airlines plane scheduled to leave Boston and a United flight that was to leave Dulles Airport in Washington DC, bringing the number of airliners implicated in the plot to six.
Both transcontinental flights were to have departed from the same airports and about the same time as the jets that slammed into the World Trade Center's twin towers, the Pentagon and a field in Pennsylvania early on Wednesday (NZ time) last week.
The development came as US Attorney-General John Ashcroft announced that the terrorism investigation, already the country's largest ever, involving up to 4000 federal agents plus police, had been expanded and the first criminal charges laid.
The American Airlines plane, Flight 43, from Boston to Los Angeles was almost identical to Flight 11, which left Boston and crashed into the World Trade Center. But Flight 43 was cancelled because of mechanical problems, possibly averting another terrorist atrocity.
Sources close to the investigation said the FBI had been unable to trace several passengers on the flight manifest who are believed to have Arabic names. They had not turned up for the rescheduled flight when air travel resumed three days later.
The United Airlines flight was cancelled at Washington Dulles airport - the airport from which the plane that hit the Pentagon departed - and federal agents are investigating reports that five male passengers who appeared to be Arabs had boarded but left quickly afterwards.
Mr Ashcroft had confirmed that agents were looking at potential hijackings which could have been thwarted.
The FBI said some of the flights were already in the air when the World Trade Center and the Pentagon were hit, but it did not say how many planes were under scrutiny.
Vice-President Dick Cheney has said that as many as six rogue aircraft were in the sky at some point that same morning.
The hypothesis is that other hijacking teams had to abort their plans after planes were grounded by the Federal Aviation Authority.
The FBI has warned that more attacks may be on the way.
Evidence of further planned mayhem was bolstered by Bob Graham, chairman of the Senate Intelligence Committee, who said the attacks might have been the start of a sustained assault on US interests.
Senator Graham told the Orlando Sentinel: "There has been very credible evidence gathered that the attacks were not designed to be a one-day event. There were other acts of terrorism in the United States and elsewhere that were part of this plan ... not necessarily hijacking another airliner but maybe putting a chemical in a city's water system or blowing up a bridge in a major urban centre," he said.
It has also been revealed by a US Government source that Mohammed Atta, suspected of being one of the hijackers on board the first plane that struck the World Trade Center, met an Iraqi intelligence officer this year in Europe.
Up to six of the hijackers, including the five men who crashed the Boeing 757 into the Pentagon, had made suburban Maryland their base in the days, perhaps weeks, leading up to the attacks.
The FBI is hunting up to 223 people in its investigation, many of whom may have given the hijackers logistical support.
Of this group, 33 were certified pilots born overseas but with US addresses, five were student pilots and 12 aircraft mechanics.
Arrests were made in Michigan when agents raided a Detroit house looking for one of the 223 wanted people.
Aided by a Federal Grand Jury in New York, investigators have detained 75 people for questioning and four others as material witnesses.
The Government also announced a new policy that gives immigration authorities 48 hours or longer in emergencies to decide whether to charge an alien with status violations - double the amount of the time they had previously.
Many of those detained over last week's attacks are being held on immigration violations.
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