However, a French-Algerian identified by the French media as a radical with "a profile similar to the kamikaze pilots" is being held for further questioning by US investigators after his arrest last month.

The German federal prosecutor said on Friday that the Iranian - who telephoned the White House warning of attacks that would "change the world order" - had no knowledge of Tuesday's attack.

"His warnings to US authorities were limited to vague claims about danger to world peace and the world order; he could not cite details," a statement from the federal prosecutor's office said.

"The attackers and the background to the attacks are not and were not known to him," it added.

The White House hung up when the man said he was in detention, but he was interviewed by German authorities and US intelligence services in the wake of the attack.

Soldier 'sans frontieres'

The French newspaper, Liberation, reported on Friday that the French-Algerian arrested in Boston was due to have been deported, but was now being held "to check any possible links with the fanatic pilots".

"He belongs to the Pakistani-Afghan network that trains Osama Bin Laden's soldiers, his best soldiers, soldiers without borders," the correspondent said, adding that all this information had been passed by French investigators to their American colleagues.

However, the US authorities did not follow up the lead, he said: "The investigation that might have prevented everything was not launched."

The 31-year-old man, who was born in St Jean de Luz, had been living in Britain before going to Boston.

Border watch

Time magazine has also reported that two of the suspected terrorists were on an FBI border watch list, but had nonetheless slipped into the United States.

The two men, who were associated with Islamic Jihad, were on the American Airlines flight which crashed into the Pentagon.

The CIA now believes the first warnings about a "spectacular" act of terror against the US came as early as June.

"A lot of this reporting we had in the summer that gained our attention and had us concerned, but wasn't specific, could have been tied to this," an intelligence officer told Time.