INVESTIGATIONS were continuing last night to find the source of an odourless pollutant that has killed thousands of fish in a North-East river.

The Environment Agency believes it knows where the pollution has come from and officials spent yesterday collecting samples to establish whether there is a case for prosecution.

Thousands of stone loach, minnows and bullheads have perished, along with hundreds of trout, lamprey and eels in the River Tame, between Middlesbrough and Stokesley, North Yorkshire.

Parents had been urged to keep children away from the poisonous river but officials said last night it did not pose as serious a threat to public health as they first thought.

James Parker, a team leader at the Environment Agency, in Darlington, said: "We are quite lucky that the river is along mainly farmland and there is no real centre of population at the site.

"We are going to continue to monitor the situation and see what happens in the watercourse and in terms of fish deaths.

"We need to carry out a little more investigation around the source to find out whether anyone is to blame and put together a case file of evidence."

The affected area spans eight kilometres from the north of Stokesley through Tanton and along the river's tributary, Main Stell.

Officials from the agency and the Government fisheries department spent yesterday collecting samples, examining aquatic life and clearing away some of the dead fish.

A biologist also took test samples of bugs and plant life at various locations on the river.

Environment management officers were sent to the area where the agency believes the pollutant originated.

The location has not been revealed so as not to jeopardise a possible prosecution, which could see the perpetrator fined up to £20,000 under the Water Resources Act.

Officials have been shocked at the high number of eel deaths, as they are usually very tolerant to pollution.

The agency was tipped off by a member of the public who spotted the dead fish.

* Anyone who knows of areas of pollution the agency should investigate is asked to call the pollution hotline on 0800 80 70 60.