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.
Varshavsky could not be reached for comment. There is no telephone listing for anyone named Varshavsky in Lawrenceville.
Tammy McLeod of Los Angeles won first place in the tournament, earning her $10,000 and a spot on the World Sudoku Team that will compete next spring in Philadelphia.