Butterball apparently has a big fat mystery on its hands: The company says it doesn't know why some of its turkeys wouldn't plump up in time for Thanksgiving this year.

CEO Rod Brenneman says in an interview with the AP that it's the first time it happened and that the company is investigating what went wrong. Butterball had announced last week that it will have a limited supply of large, fresh turkeys that are 16 pounds or heavier for the holidays.

Butterball, a privately held company based in Garner, N.C., declined to say whether it made any changes to its feed formula this year. But the problem seems to have come up rather recently.