SALT LAKE CITY -- Man-made chemical compounds known as PCBs are showing up in fish at Utah Lake, but it's still unclear where they're coming from.

The Utah Division of Water Quality had hoped to have some answers this fall about whether sediment along the lake's bottom was a source for the PCBs showing up in the fish.

"We were trying to find a smoking gun," John Whitehead, branch manager at the Division of Water Quality, said Tuesday.

Instead what they found were relatively low levels of PCBs in the sediment and no firm idea about where fish in the lake are picking them up.

PCBs -- which stands for polychlorinated biphenyls -- are chemical compounds once widely produced for industrial and commercial applications. They were frequently used as coolants and lubricants in transformers and other electrical equipment.

Despite being banned in the 1970s, PCBs still show up in the environment, including at Utah's largest natural freshwater lake.

In 2006, state health officials warned against eating too much Utah Lake carp. PCB levels in the fish exceeded Environmental Protection Agency standards but not those set by the Food and Drug Administration.

Last year, the advisory was expanded to include channel catfish.

This summer, the Division of Water Quality's Sandy Wingert set out in the hopes of finding the source.

Sediment samples were taken from 23 spots in the lake, including near Geneva Steel, Timpanogos Waste Water Treatment Plant and Saratoga Springs -- places where industrial and municipal waste water is discharged.

PCBs in all of the samples were lower than expected, Wingert said. They were also lower than a level at which more study would be needed.

So if the fish aren't being contaminated by PCBs in the lake bottom, where are the compounds coming from?

"We've got to scratch our heads and say 'Is there another source out there we should be looking for?' " Whitehead said.

One possibility is that PCBs, which degrade slowly, are simply being passed from one species to another in the lake's food chain.

"But we don't have anything to really prove this hypothesis," Wingert said.

The fish consumption advisory for catfish and carp caught at Utah Lake remains in effect.

State officials don't plan any more sediment tests at the lake. Instead, they'll focus on testing fish tissue. The next round of testing is expected in 2012.

The issue of PCBs in the fish creates a snag in helping to save another Utah Lake resident: the endangered June sucker.

Biologists say the sucker has struggled to recover, in part, because of millions of carp in the lake. Carp feeding along the lake bottom rip out weeds, which provide hiding places for young June suckers.

Wildlife officials are hoping to remove millions of carp from the lake but worry that the state's PCB advisories will make it hard to market the carp for human consumption.