Multiple Powerball winners: what if 1+ don't claim their share?

There were three winning tickets for Wednesday’s powerball drawing. As of this moment, two of them have claimed their shares.

If the third doesn’t come forward within the required window of time, what happens to that 1/3 share? Does it get divided among the two existing winners so that they each get half of the prize, or does it get folded back into the prize for future drawings?

http://www.powerball.com/pb_contact.asp

For me that infers any unclaimed prizes for shared prizes are retained by the lottery.