A third unexplained pollution incident on a south Wales river has killed more than 20,000 fish.
The detergent created a white foam which was deadly for fish
Experts are trying to trace the source of the detergent which has devastated the fish population but left other forms of wildlife unharmed.
Four miles of the River Ebbw, between Crumlin and Cwmcarn in Caerphilly borough, are hit. A misconnection in a drainage culvert is suspected.
Similar incidents in October 2003 and August 2004 caused 5,000 fish deaths.
Environment Agency investigators were on the scene within 20 minutes of the alarm being raised on Monday but have been unable to trace the source of the pollution which has killed 22,000 fish.
They do not suspect the discharge - from a culvert which drains into the river - was malicious but have appealed for help from the public in trying to trace the route of the mystery chemical.
This is the most serious pollution on the river since October 2003
Two industrial sites, each with a number of large and smaller factory units have drainage systems linking to the watercourse, as well as several residential areas.
John Gregory, who is leading the agency's investigation, said: "It's heartbreaking to see so many fish killed for the second time in a year.
He said the culvert drained surface water from above Crumlin and it was thought a misconnection was letting waste which should go to the foul sewer into the surface water system and straight into the river.
He said: "We know that this stuff is pretty powerful and it's likely that it's being disposed of innocently but with a devastating effect on the river."
The agency said the pollution would have been more devastating if it had happened during the hot weather two weeks' earlier.