The focus is on Eugene Varshavsky of Lawrenceville, N.J., who placed third out of 646 competitors and took home a $3,000 prize on Saturday.
Officials say Varshavsky blazed through the second round but couldn't finish the easy first steps in the championship puzzle. They say that raised a red flag.
Tournament spokesman Jay Devine said officials expect to complete their investigation in a few days.
Philadelphia will be the host of the World Sudoku Championship in April.
Varshavsky could not be reached for comment. There is no telephone listing for anyone named Varshavsky in Lawrenceville.
On the Web:
World Puzzle Federation: http://www.worldpuzzle.org/