tracing e-mail under a yahoo account

…here’s the situation:
an intruder logged on to someone’s yahoo mail account, changed the original password, and proceeded to send out derogatory messages to everyone in the owner’s address book…the original owner didn’t know until he started to receive phone calls…he can’t get back into this account (password changed by intruder) and has since created a new one…is there any way to find out the culprit in cases like this…any help will be greatly appreciated…