Quite possibly the efforts of the US government using illegal methods of warfare.
Its not uncommon for the FBI, NSA, or DEA, to hire 'Ethical' hackers to attack certain sites, hack certain computers, recover certain data from certain non guilty criminals, to prove their guilty-ness...
but I wouldn't know about any of this.
My suggestion, don't deal with that, simply refute the attack (DOS are easy to defend against, even in large numbers...from my understanding) and when sending the refuted attack, hide in a sniffer packet to get their addresses, go from there.