I think it’s a lot more likely that they fell into a ditch in the dark than that two armed Border Patrol agents were ambushed by deadly rock-throwing drug smugglers and that the entire FBI and every other law enforcement organization is either covering up for them or too incompetent to find evidence of this ambush.
1. "ran into" (which is alleged to have been said by one of the agents) the culvert means something totally different than "fell into" (which is not alleged to have been said) the culvert.
2. The FBI can not be fully trusted across the board.
3. The FBI now admits they don't know what happened.
Therefore, it's possible border patrol agents were attacked by illegal aliens in our country. Wouldn't be the first, nor the last time it's happened.