There could be other possibilities. Perhaps they tried to bring it up in court and weren't allowed to. There have been multiple reports that certain agents weren't allowed to testify, others were given immmunity to testify, information relative to the violent border environment or the history of the agents was not allowed, etc. Time will tell--hopefully.
---
Austin American Statesman. Austin, Tex.: Feb 17, 2006. pg. B.3
(snip)
EL PASO
Judge restricts border testimony
Lawyers in the case of two Border Patrol agents accused of shooting a suspected drug smuggler and covering it up must ask a judge before they can mention other border incidents, including a Jan. 23 standoff in which men carrying weapons and wearing Mexican military uniforms helped suspected smugglers escape to Mexico.
Ignacio Ramos and Jose Alonso Compean have been charged in the shooting of Osvaldo Aldrete Davila, a Mexican national, during a scuffle last February in Fabens. Jury selection begins today.
Prosecutors last week asked federal Judge Kathleen Cardone to exclude mentions of border confrontations and incidents of border violence. This week, Cardone said lawyers would need to ask her before any such mentions could be made.