Washington - John Walker Lindh and other al-Qaeda and Taliban prisoners told US interrogators the September 11 hijackings were supposed to be the first of three increasingly severe attacks against Americans.
But their claims could not be corroborated by government officials.
Lindh will be sentenced on Friday, likely to 20 years in prison, for supplying services to the Taliban and carrying an explosive during commission of a felony. He says heard some of the claims while serving in a 20-man Taliban infantry unit of Arabic speakers in Afghanistan, according to people familiar with his account.
Authorities have gathered similar information from prisoners of the terrorist network. But officials say the United States hasn't found specific plans for two additional large-scale attacks and they suspect the claims could involve disinformation or folklore that circulated among low-level terrorists and Taliban soldiers after September 11.
"We have not been able to corroborate the claims among the thousands of pages of documents and other evidence we have gathered the last year," one senior law enforcement official said. "We believe some of these prisoners may have been trained to give misinformation or simply were passing on rumors."
One law enforcement official said some al-Qaeda and Taliban prisoners said the second and third wave attacks could involve biological, chemical or radiological weapons to increase casualties and were designed to paralyse Americans with fear and cripple the economy.
Details of Lindh's extensive interrogation, part of his plea agreement, remain secret. However, Rohan Gunaratna, a terrorism expert who worked with defence lawyers and interviewed Lindh, said the Californian told him he picked up battlefield rumours about two waves of post-September 11 attacks.
Reading from his interview notes, Gunaratna said Lindh told him: "The original attack plan was in three phases, totalling 20 separate attacks. The first phase was... two attacks on the World Trade Centre, an attack on the Pentagon and a third attack on the White House."
The notes also reflected that Lindh said: "The second phase of attacks was going to be using biological agents and also attacks on natural gas and nuclear infrastructure.
"The second phase was going to make the US forget about the first phase. The third phase was to finish the US and was to take place within the next six months (after September 11)."
Gunaratna said that while Lindh used the word "biological", he believes from other sources that the weapon could be a radiological device, a so-called dirty bomb.
Gunaratna spoke with Lindh in his jail cell for eight hours on July 25-26 as a defence consultant, and submitted a report to a federal judge that concluded Lindh never swore loyalty to Osama bin Laden and al-Qaeda.
Still, Gunaratna said, Lindh would be a valuable US intelligence asset because he understood what makes Islamic fundamentalists join conflicts around the world.
Lindh also said he heard that 50 people were going on 20 suicide missions, but added he received the information on the front lines in October - not prior to September 11 when at a training camp, as his original indictment indicated.
Officials have had indications that additional attacks may have been planned immediately after September 11.
For instance, shortly after the jetliner crashed into the Pentagon, German intelligence intercepted a phone call from the United States suggesting other terrorist teams were on the ground and ready to strike, US and foreign intelligence officials say.
Officials said prisoners from the war on terrorism, including some kept at Guantanamo Bay, Cuba, have given similar accounts about two more attacks that were supposed to follow September 11.
The details of the prisoners' account vary widely, officials said, but most agree that the subsequent attacks were supposed to be more severe than the September 11 attacks that levelled the World Trade Centre, damaged the Pentagon, crashed a plane in Pennsylvania and killed more than 3 000.
Lindh, 21, pleaded guilty July 15. He was captured last December with other Taliban in Afghanistan, the last stop on his journey from a teenage convert to Islam in San Francisco's suburbs to a foot soldier for the vanquished Afghan regime. - Sapa-AP