NEW YORK (Reuters) - Aviation companies may not interview six current and former Federal Bureau of Investigation agents who worked on government probes into the September 11, 2001 hijacked plane attacks, a judge ruled on Thursday.

The moon rises over lower Manhattan as the "Tribute in Lights" illuminates the sky on the seventh anniversary of the attack on the World Trade Center in New York September 11, 2008. REUTERS/Gary Hershorn

The ruling in extensive litigation following the attacks addressed three wrongful death cases and 19 property damage cases, leaves several issues remaining and set July 28 for the next pretrial conference.

U.S. District Judge Alvin Hellerstein in New York wrote that the aviation defendants, “wish to argue at trial that the terrorists would have succeeded in their plans and caused the damage of which plaintiffs complain regardless of the Aviation Defendants negligence.”

They include UAL Corp, US Airways Group Inc, Delta Air Lines Inc, Continental Airlines Inc, AirTran Holdings Inc and Boeing Co.

A lawyer for the aviation companies could not immediately be reached for comment on the ruling.

Another defendant in the overall litigation is The Port Authority of New York and New Jersey, which owns the lower Manhattan site where the famed World Trade Center twin towers stood and were destroyed in the attacks.

Members of al Qaeda hijacked four commercial passenger airliners, crashed two into the twin towers, a third into the Pentagon in Washington and the fourth crashed into a field in rural Pennsylvania. Nearly 3,000 people, including all 19 hijackers, were killed.

“Permitting an inquiry into what fragments of information various government agents knew, or should have known, and at what time, but did not tell the defendants, threatens thoroughly to confuse and prejudice the jury, distract from the major issues of the case, and add to the trial substantial expense and delay,” the judge wrote.

He allowed some testimony of two FBI agents from the trial of Zacarias Moussaoui, who is serving a life sentence for conspiring with the hijackers.

A motion to admit the 9/11 Commission Report, published in 2004 and ordered by Congress, was denied except for a chronology of the report.

The case is In Re September 11 Litigation 21-MC-101, 07-7051 and 08-10646 in U.S. District Court for the Southern District of New York (Manhattan)