The Belize / Mexico border is very porous. If someone wanted to walk from Belize to the U.S., they could make it just fine.
Maybe that's what Eric Rudolph did! He walked from under the watchful nose of our crack FBI and just walked all the way to Belize?
Why is our FBI involved in anything international? I thought that is the CIA's role. It's what they are trained for. The FBI is trained for domestic stuff like finding Eric Rudolph.