Essentially, the strange circumstances of the claim for the winnings was investigated, and that trail led to enough suspicion that they tracked down the surveillance video of the ticket purchase. They released the video publicly to help identify the purchaser and were subsequently pointed to Tipton.
http://www.desmoinesregister.com/story/news/2015/04/11/prose...
Essentially, the strange circumstances of the claim for the winnings was investigated, and that trail led to enough suspicion that they tracked down the surveillance video of the ticket purchase. They released the video publicly to help identify the purchaser and were subsequently pointed to Tipton.