Here's what our lawyer tells us about cyber crimes at a federal level...
The main federal computer crime statute is 18 U.S.C. Section 1030. It prohibits, among other things, "knowingly caus[ing] the transmission of a program, information, code, or command, and as a result of such conduct, intentionally caus[ing] damage without authorization, to a protected computer" Web servers in interstate commerce are within the definition of "protected computer", and (to my recollection) service down-time is "damage" even if the computer is not physically damaged.
There are common law civil claims that could be brought against a hacker, such as trespass to chattels and interference with contract.
The threat to take down your site is a serious matter on its face. I would suggest you ask for the cybercrimes unit at the branch office of the FBI.