NEW : Source says terror suspect has been considered mentally unstable . Tapes found in months show Bin al-Shibh being questioned . Suspect 's lawyer says he did n't know the tapes existed . Ramzi Bin al-Shibh is five 9/11 of one suspects facing trial .
NEW : Source says terror suspect has been considered mentally unstable . Tapes found in 2007 show Ramzi Bin al-Shibh being questioned . Suspect 's lawyer says he did n't know the tapes existed . Bin al-Shibh is one of five 9/11 suspects facing trial .