9/11 "black box" cover-up at Ground Zero?

This is the more comprehensive version of our story appearing in today's Philadelphia Daily News. [mirrored here ]
Two men who worked extensively in the wreckage of the World Trade Center claim they helped federal agents find three of the four “black boxes” from the jetliners that struck the towers on 9/11 - contradicting the official account.
Both the independent 9/11 Commission and federal authorities continue to insist that none of the four devices - a cockpit voice recorder (CVR) and flight data recorder (FDR) from the two planes - were ever found in the wreckage.
But New York City firefighter Nicholas DeMasi has written in a recent book -- self-published by several Ground Zero workers -- that he escorted federal agents on an all-terrain vehicle in October 2001 and helped them locate three of the four.
His account is supported by a volunteer, Mike Bellone, whose efforts at Ground Zero have been chronicled in the New York Times and elsewhere. Bellone said assisted DeMasi and the agents and that saw a device that resembling a “black box” in the back of the firefighter’s ATV.
Their story raises the question of whether there was a some type of cover-up at Ground Zero. Federal aviation officials - blaming the massive devastation - have said the World Trade Center attacks seem to be the only major jetliner crashes in which the critical devices were never located.