The Port Authority released the tape to the Fire Department after top fire brass agreed to keep its contents under wraps at the behest of federal prosecutors in Virginia, who plan to use a copy at the trial of accused "20th hijacker" Zacarias Moussaoui.

"This is an important case and we don't want to jeopardize it," said a senior fire official.

There was some hope the tape would answer lingering questions about how high fire companies got in the towers, the problems they encountered and how many heard the evacuation order issued before the towers fell.

Surviving firefighters have said they didn't hear the order - which some officials attribute to a faulty radio signal-boosting repeater mounted in a nearby building.

FDNY Capt. Pete Gorman, head of the Uniformed Fire Officers Association, said he was troubled that the department didn't arrange to listen to the tape earlier, while recovery efforts were still underway at Ground Zero.

"If they had heard a certain ladder company was on a certain floor of either tower, that might have told us where we could find them in the ruins," he said.

Gorman also was puzzled about the need for confidentiality, noting, "This is chitchat from firefighter to firefighter."

Moussaoui's fired public defender agreed, and said he doesn't believe the confidentiality order applies to New York.

"It doesn't prevent the owner of the information - if they want - from giving it out," said Frank Dunham.