DONNA, Texas (AP) -- Environmental officials will soon begin extensive testing at a South Texas reservoir and canal where people continue to fish despite warnings about toxic contamination.

Residents near the Donna, Texas, reservoir are being warned in door-to-door canvassing about the risks of eating the fish, which are contaminated with polychlorinated biphenyls or PCBs.

A carp caught in the canal in 1993 had the highest PCB levels ever recorded in a fish.

The testing is a step toward a possible cleanup effort.

The site is on the Environmental Protection Agency's priority list.

Searches have failed to identify the source of the chemicals, but investigators believe a piece of equipment may have been dumped in the canal long ago.