A very large percentage have criminal histories that caused them to be rejected. I should point out, however, that when a transport worker is initially rejected by TSA they have the full ability to appeal and to challenge that decision. They involve their state jurisdiction in that process to see if they can mitigate the situation, to see if there was incorrect information and so forth. A very large percentage--and I'm not going to say 80% or 90%, but I believe it is very high, at least 80%--was due to the criminal history this man or woman had and therefore is disallowed to transport.
On March 5th, 2009. See this statement in context.