A clean-up operation is under way after a pollutant was dumped in a Wrexham brook.
Thick brown sludge, thought to be animal fat, spread through 50m (164ft) of a small stream feeding the Redwither Brook on Wrexham Industrial Estate.
Environment Agency Wales officers are appealing for information, saying the clean-up will cost thousands of pounds.
"It is unusual as this is not really waste, but a useful product that has a market value," said a spokesperson.
The sludge was discovered on 10 February and contractors have been working to clear it.
Initial investigations have led them to believe that the substance is tallow or rendered animal fat, which was dumped down one of the banks into the stream.
Officers want to speak to anyone who may have seen something suspicious on the Bridgeway Centre Industrial Estate, just off Coed Aben Road.
A spokesperson from Environment Agency Wales said: "Now that we have identified what the sludge is we can narrow our search for those responsible.
"A local business may have been expecting a delivery and it did not arrive.
"Unfortunately, this clean-up will cost us, and eventually the taxpayer, between £3,000 to £4,000, so we are very keen to find those responsible."
Witnesses are asked to call the emergency hotline on 0800 807060 and quote incident reference number 857364.