BLYTHE (AP) — Federal wildlife officials are investigating as many as 60 bird deaths at a Southern California solar plant.

The birds apparently got caught up in an oily substance in evaporation ponds at the Genesis solar plant west of Blythe near the Arizona border, said U.S. Fish and Wildlife Service spokeswoman Jane Hendron.

It’s not yet known what species of birds died.

“We are still gathering facts,” Hendron told the Riverside Press-Enterprise on Thursday.

Between 50 and 60 birds were found dead Monday in two ponds, according to Steven Stengel, a spokesman for NextEra Energy Resources, which operates the plant. The company is cooperating with the investigation, he said.

Stengel said he knows of no oil or chemical spills at the 1,950-acre plant. He said the plant was operating normally this week.

The plant uses 620,000 curved mirrors, called thermal troughs, to collect solar energy. The mirrors focus sunlight onto pipes containing a pressurized, heat-absorbing fluid. The fluid gets as hot as 739 degrees before it is piped to a heat transfer unit, where it boils water to make steam.

Ileene Anderson, a biologist for the Center for Biological Diversity, said that, before the project’s approval, state and federal officials had discussed covering ponds at the plant with netting to keep birds out. The concern then was that birds attracted to ponds could crash into mirrors, she told the newspaper.

It was not clear Friday why birds had access to the ponds.

Birds in the area include ducks, grebes, coots, rails and pelicans, Anderson said.