Further water samples are being taken to find the source of pollution in a river which killed hundreds of fish.
Specialists returned to Afon Llynfi, near Pontithel, Powys, on Sunday to assess how wider biodiversity has been affected on the River Wye tributary.
Natural Resources Wales (NRW) said 200 bullheads had been found dead.
It said the “water is slow-moving which means that pollutants are still present and potentially still affecting wildlife”.
As well as bullheads people said they had seen many more dead fish, including brown trout, with “crayfish crawling up the bank to escape the water”.
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In a statement, NRW said: “Our officers are on site again today to continue the investigation to determine the source of the pollution.
“We’re taking more water samples and carrying out a biology assessment of invertebrates in the affected stretch of the river.”
NRW said it believes “no further pollution is entering the river” .
People are advised not to swim in the river or nearby waters “until it is safe to do so”.
Public Health Wales has also been informed.