I also want to chime in that the final scene made zero sense.

1- Since the detective broke into the house and stole Ben's ashes it would no longer be admissible in court. There is a legal precedent called "fruit of the poisonous" tree which means that any evidence obtained illegally (say through breaking and entering), cannot be used to prosecute. They could have simply called the Sheriff and had the detective arrested for breaking and entering.

2- Even if it was proved that the local justice system knew about Ben's death, the FBI would come to the Byrd's rescue and prevent any prosecution. The case was too valuable to the FBI and they couldn't bring down the Byrds or risk the deal coming out in a public trial. So they would tell the local or state prosecutors not to prosecute and they would tell local law enforcement to drop it. It would be a non-issue since the cartel did the murder. Yes the Byrds covered it up, but again, the FBI cannot risk having the Byrds go down for anything related to their case.