I have seen airplanes visually prior to getting any indication on TCAS. HOW/WHY? Possible transponder problems, possible blanking of tranponder antenna by wing or other part of plane, indeed some cargo planes only have transponder antenna on bottom, I followed a cargo DC10 for 3 minutes with nothing on TCAS...this may have changed since then as even cargo planes are getting better equipment.
one post indicating the biz jet wing hitting a vital surface is a thought...perhaps too if the biz jet wing hit the canopy/cockpit of the boeing, killing both boeing pilots, add decompression, other failures including an autopilot disengagement and bang.
I didn't see altitudes published if anyone has them...and if there was a collision and RVSM is a component of the cause, maybe we should go back to 2000' at higher altitudes for seperation.
sad crash
jon