Employees at a Seattle-area thrift shop were left scratching their heads after someone donated three human skulls.

Now, Washington state officials are searching for whoever donated the skulls to get more information.

The King County medical examiner said in a statement on Wednesday that two of the skulls were from adults and appeared to have been used in a medical clinic or for instruction.

The third is very old and appears to have belonged to a Native American child.

State law requires the Native American skull be returned to its tribe. The medical examiner is asking whoever donated the skulls to provide some details, without penalty, so that can happen.

Spokesman Keith Seinfeld said Thursday there had been only one call about the skulls and it wasn't helpful.

The skulls were donated last month to the Goodwill store in Bellevue, Wash., a suburb about 20 kilometres from Seattle.