The team investigating a stomach bug outbreak in north Wales has said the Cwellyn reservoir is "the probable source" of the illness.
A total of 208 people have been infected with the water-borne parasite, cryptosporidium, since 1 October.
Three quarters of people who had fallen ill as a result of the bug were found to live within the Cwellyn supply area.
But tests of the water continued to show that levels of cryptosporidium were within regulatory limits.
A spokesman for the outbreak control team said that the cryptosporidium parasite previously found in water from the Cwellyn reservoir, had been found to be the same strain as that isolated from people who have fallen ill in the area.
A team spokesman said they had found five ways that contamination from human sewage may have entered the reservoir.
But he said it was "unlikely that any one of these will ever be confirmed as the source of the outbreak".
In a statement the outbreak control team said that the risk of illness was greatest in people who drank large amounts of unboiled water.
On Wednesday, Welsh Water said it would compensate 37,000 customers advised to boil drinking water to avoid the stomach bug.
The company said the £25 payments, to be made in the new year, would make up for energy costs and the inconvenience.
Residents wishing to find out whether their home's supply is affected can check their postcode against a list held by Dwr Cymru Welsh Water at any time by ringing 0800 052 0130.
NHS Direct is operating two help lines on 0845 850 9850 and 0845 600 6086 between 0900 GMT and 1800 GMT on weekdays, and 0900 GMT and 1700 GMT at weekends.
But Welsh Water said the payout did not mean the water supply was to blame, as investigators are yet to find the source of the illness.
The warning to boil water was issued to 37,000 households - around 70,000 people - on 29 November. The notice will be in force until at least 9 January.
On Friday, the "all clear" notice was issued to 9,000 households after their water was switched from the Cwellyn treatment works.
But Welsh Water said payments would be made to all customers who had been advised to boil water during the cryptosporidium outbreak.
The company is writing to the affected households with details of the compensation.