The state Department of Environmental Protection says initial tests of the water at Ridley Park Lake show nothing abnormal after about 1,000 fish were found dead there late Saturday.
DEP spokeswoman Deborah Fries says a pollutant could have killed the fish and been diluted by rain by the time testing began at the Delaware County lake.
Fries says the DEP is mapping the sewers in the area to see if that could have played a role. Water samples are also being sent to Harrisburg for further testing.
But Fries says investigators may never know what caused the fish to die.
Fishing is discouraged for the time being.
