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SAN DIEGO — The last of three California condor chicks found dead this month died of unknown causes, officials said Thursday. 

Results of a necropsy performed on the chick at the San Diego Zoo were inconclusive, said Bruce Palmer, California condor recovery program coordinator for the U.S. Fish and Wildlife Service. 

The chick was found dead Tuesday in the Los Padres National Forest, just weeks after the remains of the other two birds were retrieved. The three were the first condors to be laid and hatched in the wild since 1984. 

Biologists are uncertain how the first chick died. The second died after it ingested a dozen bottle caps and multiple shards of plastic and glass it likely found in its nest. 

During the necropsy, a few pieces of either plastic or rubber were found in the gizzard of the third chick, but are not thought to have killed the bird, Palmer said. The birds probably mistook the objects for bone fragments, which they eat to provide much needed calcium, he said.