Just read that new information indicates military radar tracked the aircraft and that the aircraft may have tried to turn back to its departure point.
http://news.yahoo.com/air-force-chie...060410114.html
If they lost power, the transponder would have stopped transmitting, the ATC radar would have lost tracking except for a "primary" target. Depending on the range, civilian radar might not have picked up the primary target without a working transponder.
Defense radar might have though... Hmmmmm
EDIT**
Appears to be confirmed. Defense radar indications at this point are said to have shown the aircraft making a turn back towards its departure point.