Jayna Davis wrote a very informative book on the subject, providing evidence of a third collaborator - a Middle Eastern man.
Nobody wanted to hear about it; still don't.
I remember the day-of, reports of a suspicious Middle-Eastern looking man fleeing the scene. This was before 9/11 - before people's suspicions were highly attuned to suspect Islamists every time something exploded.
The reports quickly gave way to the WASP right-winger, Timothy McVeigh, and the reports of the Middle-Eastern man went down the memory hole.