AN INVESTIGATION is still under way to find out why 400 fish died in a river in Ravensthorpe.
Environment Agency experts said tests confirmed a massive silt deposit had led to the fish being found dead last week.
The agency does not believe the silt was stirred up naturally from the river bed.
It is possible that it was put into the river deliberately.
Roach, perch, dace and chub died in the stretch of water where the River Spen joins the River Calder at Ravensthorpe.
Further tests are being carried out on samples from the river to find out how the silt killed the fish and to trace its source.
The stretch of water has spent five years recovering from frequent past pollution incidents.
The amount of pollution incidents dropped from 60 or 70 a year to none in the past year, with the help of a clean-up campaign by the Environment Agency, Kirklees Council, Yorkshire Water and local businesses.