Police are scratching their heads trying to work out who donated a human skull to a thrift store in Austin, Texas.

Homicide Detective Derek Israel said foul play is not suspected and that he simply wants to know how the adult skull came to be left at the Goodwill store.

Goodwill store staff discovered the skull while sifting through donations on July 16. A spokeswoman said it could have been gifted up to a week prior to its discovery.