NEW YORK, US - Aviation companies sued by the families of Sept 11 victims for failing to safeguard air travel are in turn blaming federal investigators - arguing the Federal Aviation Administration (FAA) was not alerted that al-Qaeda was poised to launch terrorist attacks.

In court documents filed this week in the US District Court in Manhattan, aviation companies are seeking to force five FBI employees to provide testimony that may help defend against claims that the companies share blame in the attacks.

'The aviation parties anticipate that the FBI witnesses' testimony will demonstrate that the FBI had information before Sept 11 indicating that al-Qaeda may have been about to launch terrorist attacks on civil aviation, which it did not timely pass along to the Federal Aviation Administration,' lawyers wrote.

The airlines and aviation companies are defending themselves against lawsuits seeking billions of dollars in damages for injuries, fatalities, property damage and business losses related to the Sept 11, 2001 terrorist attack.

The companies in turn filed separate lawsuits against the CIA and the FBI last August to force terrorism investigators to tell whether the aviation industry was to blame for the Sept 11 attacks.

The latest documents filed by the airlines, airport authorities, security companies and an aircraft manufacturer argue that if the FAA had known about an FBI investigation of Zacarias Moussaoui weeks before the Sept 11 attacks, it could have amended security measures to guard against the type of terrorist attack Moussaoui was planning.

The aviation defendants said the FBI has refused to permit even a single deposition, although the agency does not deny that five potential witnesses in the case have already testified and made other public statements before the 9/11 Commission, the Moussaoui trial jury and the media.

In the lawsuit against the CIA, companies including American Airlines Inc, United Airlines Inc, US Airways Group Inc, Delta Air Lines Inc, Continental Airlines Inc and The Boeing Co are seeking to interview the deputy chief of the CIA's bin Laden unit in 2001, and an FBI special agent assigned to the unit at that time. The names of both are secret.

The lawyers said testimony from both agencies was critical to their defence.