Nobody here said DEA hired him, but it is true that a prosecutor would only in very exceptional circumstances use a private investigater ... the govt. has plenty of people they could use outside of the prosecutor and police.
Now ... it is true that the call may have been made by a govt. investigator or cop who said he was private ... but it was not true.
Also, a private investigator can be hired by the defense attorneys.
Either way, govt. or private, they may not be truthful in what they tell you.
Sounds fishy that they did it by phone also. Very fishy. almost any real investigator would come in person, preferably suprising you. They judge your non-verbal reation as well as what you say. Hard to do over the phone.
Then, on the other hand, in a criminal case, the evidence would usually be sealed until the trial. So the *only* ones who have access to those customer lists would be the prosecution and the defense. Yes, if it's the prosecution's evidence they do have to share it with defense counsel.