NEW : Source says terror suspect has been considered mentally unstable . Tapes found in 2007 show him talking about killing people and blowing up buildings, but he was not on any watch lists at the time of his arrest. Suspect 's lawyer says he did n't know the tapes existed . An October 2007 letter from federal prosecutors to U.S. District Judge Leonie Brinkema confirmed the existence of the tapes . Bin al-Shibh is one of five 9/11 suspects facing trial .
NEW : Source says terror suspect has been considered mentally unstable . Tapes found in 2007 show him talking about killing people and blowing up buildings, but he was not on any watch lists at the time of his arrest. Suspect 's lawyer says he did n't know the tapes existed . Bin al-Shibh is one of five 9/11 suspects facing trial .