THE FINAL three minutes of hijacked United Flight 93 are still a mystery more than a year after it crashed in western Pennsylvania - even to grieving relatives who sought comfort in listening to its cockpit tapes in April.

A Daily News investigation has found a roughly three-minute gap between the time the tape goes silent - according to government-prepared transcripts - and the time that top scientists have pinpointed for the crash.

Several leading seismologists agree that Flight 93 crashed last Sept. 11 at 10:06:05 a.m., give or take a couple of seconds. Family members allowed to hear the cockpit voice recorder in Princeton, N.J., last spring were told it stopped just after 10:03.

The FBI and other agencies refused repeated requests to explain the discrepancy.

The cockpit voice recorder a roughly 30-minute tape loop, is supposed to record the sounds inside the cockpit right up until the moment of impact and usually does.

Aviation experts said there could be several explanations for the gap.

They said it could mean that the FBI and other government agencies either failed to properly synchronize the times, or there were other problems in the retrieving or handling of the tape from the so-called "black box" recovered from the wreckage at Shanksville, Pa.

Or, experts speculated, it could mean there was a major on-board electrical failure on the plane three minutes before Flight 93 crashed, causing the recorder to quit working.

What's not told

The broader significance is that the three-minute gap points to how little is really known about how and why Flight 93 crashed - even as the saga of the doomed jetliner and cell-phone calls from some of the 40 passengers and crew continue to captivate the nation.

"That's part of the whole war aspect - we don't want to tell about what we did and didn't do," said Vernon Grose, a former National Transportation Safety Board member who says he still has questions about the Flight 93 crash. He said he doubts there will ever be "a nice, open public hearing with eyewitnesses telling what they saw."

However, in recent weeks, two books about Flight 93 have topped the best-seller lists, while President Bush and other top government officials continue to invoke the story - based largely on the cell-phone calls - of fighting between the passengers and the hijackers as a "Let's roll" rallying cry to continue the war against global terrorism.

But the FBI has clamped a tight lid of secrecy on the flight data recorder - which could best show how Flight 93 actually crashed - and on the cockpit voice recorder.

"We have no comment at all on the tape issue," said Sam Dibbley, spokeswoman for the U.S. Attorney's office in northern Virginia that presented the tape to families.

An FBI spokesman, Steven Berry, said the bureau continues to officially list the time of the Flight 93 crash as 10:03 a.m. The NTSB referred all questions to the FBI.

But the relatives of Flight 93 passengers who heard the cockpit tape April 18 at a Princeton hotel said government officials laid out a timetable for the crash in a briefing and in a transcript that accompanied the recording. Relatives later reported they heard sounds of an on-board struggle beginning at 9:58 a.m., but there was a final "rushing sound" at 10:03, and the tape fell silent.

What can be heard

"There is no sound of the impact," said Kenneth Nacke, whose brother, Lou Nacke Jr., is one of the passengers believed to have fought with the hijackers. Nacke confirmed that the government said the tape ended at 10:03 a.m.

He added: "The quality of the sound is really poor."

Vaughn Hoglan, the uncle of passenger Mark Bingham, said by phone from California that near the end there are shouts of "pull up, pull up," but the end of the tape "is inferred - there's no impact."

New York Times reporter Jere Longman, who spoke with relatives of all but one of the 40 Flight 93 victims, writes in the epilogue to bestseller "Among the Heroes" that "at about three minutes after ten, the tape went silent."

Lisa Beamer, the wife of passenger Todd Beamer, who heard the tape while working on her No. 1 best-seller "Let's Roll," also gives 10:03 as the end of the flight.

Seismologists - experts in the earth's vibrations - have almost exactly pinpointed the time of the crash of Flight 93 at 10:06:05.

"The seismic signals are consistent with impact at 10:06:05," plus or minus two seconds, said Terry Wallace, who heads the Southern Arizona Seismic Observatory and is considered the leading expert on the seismology of man-made events. "I don't know where the 10:03 time comes from."

Likewise, a written study commissioned by the Department of Defense - carried out by seismologists from Columbia University and the Maryland Geological Survey - also determined impact was at 10:06:05.

Normally, such a large discrepancy might be cleared up when the National Transportation Safety Board releases a written transcript of the voice recorder - edited for sounds of suffering or profanity - right before holding public hearings on an air disaster. But because the Flight 93 crash was part of a criminal act, no NTSB hearings are expected.

The Justice Department has also insisted that the cockpit tape can't be released because it will be played to the jury at the trial of admitted al Qaeda terrorist Zacarias Moussaoui, now set for January.

Although Moussaoui is often referred to in the media as "the 20th hijacker," there's been no evidence that he was slated to be on board Flight 93 or the three other planes hijacked on Sept. 11. Moussaoui's court-appointed lawyers sought last week to block the use of the recording.

What could've happened

Last fall, as the saga of the Flight 93 passenger uprising became widely known, several relatives of the crash victims made an unusual request: They wanted to hear the actual tape. The FBI initially issued a cold refusal.

"While we empathize with the grieving families, we do not believe that the horror captured on the cockpit voice recording will console them in any way," FBI Assistant Director John Collingwood said last December. But under continuing pressure, the bureau changed its mind and agreed to the unusual April gathering at a Princeton Marriott hotel.

None of the family members interviewed for this story recalls any explanation of a discrepancy between the times on the tape recording and the actual crash at 10:06.

They were, according to the relatives and published accounts, given a talk by one of Moussaoui's prosecutors, who speculated that the passengers may have used a food cart to break into the cockpit.

But with government officials refusing to be interviewed, leading aviation experts interviewed for this story could only speculate about the tape discrepancy.

Possibilities they suggested:

• The FBI could have bungled this part of the investigation by failing to synchronize the time stamp of clocks onboard Flight 93 - which could have been set wrong - with air traffic control tapes and other tones that make it possible to determine the exact, correct times. Such a mistake would mean that the tape really did run until the impact, but that all the times given to the relatives on the transcript were off by three minutes.

Investigators typically nail down the correct times very early in a probe, experts said. Todd Curtis, who runs the Web site AirSafe.com, said the three-minute gap "does not make sense."

"From what I have heard about the flight's CVR [cockpit voice recorder], there was at least one transmission from the cockpit to air traffic control that would have been captured by the ATC tapes," Curtis said. "Those tapes should also have some kind of time reference."

• At 10:03, the hijackers - or possibly passengers and crew who were fighting to regain control of the plane - flipped a circuit breaker or switch that cut off power to the cockpit voice recorder.

Experts said this would explain why the tape ends abruptly, but they had no idea why the terrorists would do such a thing, especially so far along into their hijacking. And they noted that the location of cockpit circuit breakers makes it unlikely it was struck accidentally during a struggle.

"That would be a much tougher task than turning off the transponder," said R. John Hansman, an aviation professor at the Massachusetts Institute of Technology. "You would have to know exactly which circuit breaker to pull."

• There was a major on-board electrical failure before the crash - although it's not clear what could have triggered this. It has happened before. On Swissair Flight 111, which crashed off the coast of Nova Scotia in September 1998, the cockpit fire that caused the crash also killed power to the plane's two black boxes six full minutes before the crash.

New evidence that came out last week may support the electrical-failure theory. A federal air traffic controller from Cleveland, Stacey Taylor, told "Dateline NBC" that Flight 93's transponder, initially shut off by the hijackers, came back on briefly only to give out - at 10:03 a.m.

• There was some unknown problem either in retrieving the cockpit tape from the black box, or in its handling by government officials and contractors since last September, or in the presentation that was given in Princeton.

No one has stepped forward with any evidence of that.

But the three-minute gap is certain to fuel ongoing debates on the Internet over how Flight 93 really crashed, and whether the plane could have been shot down by military jet fighters that were sent aloft as the Sept. 11 hijackings unfolded. The government insists there was no shootdown.

Numerous witnesses in the Shanksville area have told the Daily News and other publications since last September that a mysterious, low-flying unmarked white jet, military in nature, circled the area at the time of the crash. The FBI has claimed this was a business jet that had been asked by air-traffic controllers to inspect the Flight 93 crater.

The debate has also been driven by the wide debris field from Flight 93 - including papers found eight miles away - and by conflicting accounts over whether a 911 caller reported an explosion and white smoke on board.

Grose, the former NTSB member, said he doubts the entire story of Flight 93 will ever be told.

"I don't think so," he said. "It's like David Crockett at the Alamo. We need heroes."