Thousands of fish have died in River Wandle in south London after a toxic spillage entered the river.
The Environment Agency has begun an inquiry into the incident which killed at least 2,000 fish along a 5km stretch of the river.
Local residents called up the agency and reported seeing fish floating on the surface between Beddington in Croydon and Merton on Monday.
Officers are trying to ascertain how many fish survived the spill.
The dead fish included barbel, chub and eels.
The agency believes plants and other invertebrate creatures may have been affected by the pollutant, although birds and mammals seem to be unharmed.
Mike Denbigh, environment management team leader, said: "This very serious incident is being vigorously investigated by the Environment Agency.
"We believe we have confirmed the nature of the pollutant that entered the stream.
"By looking at the fish in the Wandle that have survived, we will be able to assess the impact and make decisions about how best to rehabilitate the river's ecology. This will be a long-term process," he added.
On Thursday, the agency's fisheries and ecology team isolated a 100m area of the river next to Ravensbury Park in Merton and used equipment which generated low voltage electric currents to check how many fish survived.
The team found 32 specimens, including carp, eels, chub and roach, alive in that area.
John Sutton, fisheries and biodiversity team leader, said: "It is clear from these early results that the pollution has had a dramatic impact on this reach. The effects of this event will be felt for many years to come."
The 14km long River Wandle flows through south London, starting at Croydon it passes through Sutton and Merton before joining River Thames at Wandsworth.
The Wandle, once dubbed a "sewer" because of waste from tanneries flowing into it during the industrial revolution, was cleaned up by environmentalists.