STRONGSVILLE -- It's being called the biggest fish kill in Northeast Ohio in more than a decade.

The Ohio EPA, Ohio Division of Natural Resources Division of Wildlife, the Northeast Ohio Regional Sewer District and the Cleveland Metroparks are all trying to find out what caused it.

The Northeast Ohio Regional Sewer District's Frank Greenwall says, "It's like looking for a needle in a haystack."

Whatever got in the water did its damage over the weekend, killing almost 29,000 fish in a three-mile stretch of the river between Strongsville and Berea.

"It's terrible...Pretty crazy. It's a good thing Idon't eat stuff out of this part of the river," said fisherman Brad Washburn.

Sewer District crews waded in the river and feeder creeks and streams looking for answers.

EPA crews visited nearby businesses and checked storm sewers. That also proved to be a dead end.

Regional Sewer Distict lab technicians were checking for ammonia that might suggest fertilizer runoff, anything that might deplete oxygen, and possibly deadly metals.

"It's not a smoking gun. It's just a first step and will help us decide where we go next," said the Sewer District's Head of Analytical Services Mark Citriglia.

Investigators would also welcome help from the public.

If you have any information about a possible spill, you should call 1-800-POACHER or 1-800-WILDLIFE.

EPA tests may take a week to complete.

Investigators admits it's possible they may not discover the cause of this incident.

The Metroparks were planning to restock this part of the river with trout. That was moved upstream.

The good news is that living fish have been found downstream near the stretch where the damage occurred, indicating it was a limited, one-time incident that is not continuing to pollute the water.

Officially, the incident is still categorized as a "Fish Kill-No Known Cause."