Investigations are continuing into the seaside incident in Essex which left beachgoers struggling to breathe but the cause may never be known.
Emergency services were called to Frinton-on-Sea over the Bank Holiday weekend after receiving several reports of people coughing and struggling for breath.
Reports also came in from Clacton and Walton-on-the-Naze, with Tendring District Council warning sunbathers to avoid the water.
The council says it is still awaiting test results but has warned they could prove inconclusive.
It says it has taken precautions and is satisfied there is no further risk. A spokesperson added: “Incidents such as these are not unheard of around the country’s coastline, and often with the tides pass quickly."
“There have been no reported incidents in relation to the seawater over the last two days. Our current guidance for the public is to use caution around the water.
A number of causes have been suggested ranging from algae, a fuel spillage and a spike in pollution and ozone levels.
Tendring Council says it has looked into a whole range of possibilities but said "it is possible we will never know for sure what caused this incident.”