Eh. I have worked with US investigators quite a few times. They really try to work with you. If you don't have the information and you tell them you don't have the information - they pretty much drop it. FBI/local police are not interested in pulling servers and doing forensics. This isn't what their prosecutors want. Prosecutors want: "Hey, give me all data you have for IP address X." If you honestly can explain to them you don't have it -- they just drop it move on to easier cases IMHO.