For those interested, are these firemen lying?
http://www.arcticbeacon.citymaker.com/articles/article/1518131/17860.htm

Honorary firefighter Mike Bellone claims he was approached by unknown bureau agents a short time after he and his partner Nicholas DeMasi, a retired New York firefighter, found three of the four "black boxes" among the WTC rubble before January 2002. Bellone is retired and was made an honorary New York fireman for his efforts after 911. DeMasi also recently retired from Engine Co. 261, nicknamed the "Flaming Skulls," after serving a brief stint after 911 with the fire department's marine unit.

"It's extremely rare that we don't get the recorders back,' said NTSB spokesman Ted Lopatkiewicz. "I can't remember another case which we did not recover the recorders."

http://www.howstuffworks.com/black-box.htm

One thing the NTSB learned from experience: be careful where you put these things. Recorders used to be located near the point where the wings joined the fuselage, the theory being that this was the most heavily constructed part of the plane. Problem was, being heavily constructed, the parts of the plane falling on the recorders often crushed them. Now the recorders are put in the tail section so that, assuming your typical crashing plane goes in nose first, the forward part of the airframe absorbs most of the impact.
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Crash impact - Researchers shoot the CSMU down an air cannon to create an impact of 3,400 Gs (1 G is the force of Earth's gravity, which determines how much something weighs). At 3,400 Gs, the CSMU hits an aluminum, honeycomb target at a force equal to 3,400 times its weight. This impact force is equal to or in excess of what a recorder might experience in an actual crash.

Fire test - Researchers place the unit into a propane-source fireball, cooking it using three burners. The unit sits inside the fire at 2,000 degrees Fahrenheit (1,100 C) for one hour. The FAA requires that all solid-state recorders be able to survive at least one hour at this temperature.