Ezra 2:61-63

and of the descendants of the cohanim: descendants of Havayah, descendants of Hakotz, and descendants of Barzillai, who took a wife from the daughters of Barzillai the Gil'adi and was named after them.

These tried to locate their genealogical records, but they weren't found. Therefore they were considered defiled and were not allowed to serve as cohanim.

The Tirshata told them not to eat any of the especially holy food until a cohen appeared who could consult the urim and tumim.