The Environment Agency is to launch an internal inquiry into its handling of a major pollution incident on a north Wales river that killed an estimated 100,000 fish.

Despite exhaustive tests, the agency has yet to discover who or what was responsible for the disaster on the River Dee.

Scientists believe it was caused by an illegally dumped chemical - or a freak act of nature.

But local fishermen said important questions need to be asked about the way the investigation was handled.

Drinking water

The River Dee is one of the most regulated and controlled rivers in Britain.

It is important - not just for its beauty or fishing - but as a source of drinking water for nearly three million people.

The pollution incident at the end of July was devastating.

Not only were approximately 100,000 coarse fish killed, but also around 200 salmon.

Investigation reviewed

Environment Agency officials believe it will be years before the river's fish stocks recover.

But despite what the agency described as exhaustive tests - including computer modelling - it has failed to discover who or what was responsible.

The internal inquiry will review the way the agency's investigation was carried out.

Fishing groups on the Dee said it should question whether staff involved were sufficiently qualified or experienced to deal with such an important incident.

Keith Allanson, chairman of the region's fishery advisory group, said it "beggared belief" that the source of the incident could not be determined.

"We thought the monitoring system was infallible," he said.

"Stocks of chub and bream could take 15 years to recover and people are asking how it is that on such a well monitored river, this sort of thing could happen."

The agency's environment protection manager Andrew Dickson said the one clear thing that caused the incident was the complete lack of oxygen in the water.

One of the focal points of the investigation is a heavy storm on the night of the incident.

"All the analysis points to de-oxygenating mud that was flushed out by very severe thunderstorms on the Friday night that then stripped the water of oxygen," said Mr Dickson.

Mr Dickson said the EA team involved had reacted quickly and efficiently to the incident, which has baffled scientists.

"We will want to pick up lessons from this."