If you want IPs, get a legal request for information, such as a court order. It's best to have a "chain of custody" so to speak when it comes to things like this. If we handed out IPs to any random who asked, and violence of some sort ended up happening, then we would be on the hook if we couldn't show a paper trail of who requested the information. If you are on the up and up and don't plan on doing anything illegal, then there shouldn't be any problems with this. Court orders (which, again, are just a legal request for information) are standard when requesting information from third parties.
Thanks for the response BadBear, would you be able to tell us if the accounts in question have overlapping log ins from the same IP? That would answer the question without disclosing any information that could cause you legal or ethical problems. There is a considerable amount of BTC riding on this decision, any help you can render would be greatly appreciated.
R.