Public health officials say it is now unlikely that the source of a stomach bug, which has hit people in Gwynedd and Anglesey, will ever be confirmed.
New treatment equipment has been installed at the reservoir
Two hundred and thirty one people have been infected with cryptosporidium.
Tests have been continuing for months to try and find the outbreak source.
The National Public Health Service said it hoped that thousands of affected homes and businesses would be able to stop boiling their water in the next fortnight.
Officials say there are five ways the reservoir which supplies the areas affected could have been contaminated.
The Llyn Cwellyn reservoir in Snowdonia has been under investigation since cases of the bug, which causes stomach upsets and diarrhoea, first emerged in October.
Some customers have had to boil water since November
After a meeting of the outbreak control team on Wednesday, a statement from the National Public Health Service for Wales said: "All the evidence and expert opinion points to Llyn Cwellyn being the probable source of infection. There is no alternative explanation."
But the statement also added: "It remains unlikely that the point source or sources of the outbreak will ever be confirmed."
Officials also said they had discovered that there had been cases of cryptosporidium infection, caused by a water-borne parasite, in the south Gwynedd and Anglesey area in early October.
But it would not give any more details, saying it has "a duty to protect the confidentiality of individuals".
Extra water treatment equipment has now been installed at the lake.
The boil water notice which was originally to have been lifted for householders on 9 January was extended earlier this month.
Around 70,000 people were first told to boil their water at the end of November
Welsh Water Dwr Cymru has said it would make £25 payments to 37,000 customers advised to boil their water to avoid the bug.
The one-off payments were to make up for energy costs and the inconvenience, according to Welsh Water.
But it said the compensation did not mean the water supply was to blame.