The Environment Agency has admitted it does not know what killed a number of fish in the River Brett in Lavenham.
Lavenham Parish Council clerk Jane Bellward said she had received reports from members of the public about the fish, which she passed on to the agency.
Speaking this week, an Environment Agency spokeswoman said staff were sent to the site, and confirmed that tests were carried out on the water to determine the cause of the deaths.
She said the results had proved inconclusive, adding that a number of factors could have been to blame.
“Sometimes, temperature changes can cause it,” she said. “There could have been a little pollution going through.”
Regular checks on the river will be made by agency officials to try to find out what caused the deaths and to see if anything similar happens again.
Residents have been asked to keep a look out for anything unusual in the river and to report any incidents to the agency hotline on 0800 807060.