From this:

Osvaldo Aldrete Davila, the drug smuggler who was shot in the buttocks by El Paso Border Patrol agents Ignacio Ramos and Jose Alonso Compean in 2005, was arrested on drug charges at the Zaragoza Bridge this morning, officials with the U.S. Attorney's office said.

Aldrete was arrested by agents of the Drug Enforcement Administration and the Department of Homeland Security Office of Inspector General on a sealed indictment charging him with conspiracy to possess with intent to distribute marijuana, and with possession with intent to distribute marijuana in September and October 2005.

That was both after he was shot (February 2005) and given immunity, and if he had been charged earlier it might have changed the outcome of the case. U.S. Attorney Johnny Sutton says it was because he'd been working with the DEA to get new evidence (link).