Investigators searching for the cause of a stomach bug outbreak in north Wales have been examining a faulty septic tank.
But they say there is no evidence linking the tank in the Waunfawr area of Gwynedd, and the nearby Cwellyn water treatment works, which is the focus of their inquiry.
So far 162 people have been affected by the cryptosporidium parasite.
Welsh Water has said customers might receive compensation.
Gwynedd Council is to serve a notice on the owner of the premises with the septic tank to carry out remedial work.
Chris Lines, a spokesman for the outbreak control team, confirmed that the "private sewage treatment system" was in the Cwellyn catchment area.
But he said public health officicials did not believe anything had leaked into the Cwellyn water treatment works, nor that the tank was the source of the cryptosporidium outbreak.
Dr Mark Walker from the National Public Health Service for Wales said several steps needed to be taken to prove there was a link between the tank and the outbreak.
"First of all we have to be able to demonstrate the human strain of cryptosporidium has got from that source into a water course and from that water course in to the reservoir," he said.
"Secondly we have to be able to demonstrate that it's reaching the reservoir in large enough quantities to contaminate the water supply.
"Thirdly we have to be able to demonstrate that it has to find its way across the reservoir into the water supply."
There has been anger by some residents at what they see as a "delay" in letting people know they need to boil their drinking water.
Katie Buxton from Beaumaris, who is eight months pregnant with twins, said she had only heard about the outbreak by chance.
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.
"I heard by accident during a chance conversation with my midwife about the need to boil water, but it was too late as I became ill last Monday. We should have been told much earlier," she said.
Meanwhile, some living outside the Cwellyn supply area are also angry after Welsh Water advised them to boil water despite living in unaffected areas.
Welsh Water said they were aware that letters had been sent to some customers not supplied by the works, but that they had mailed across a wide area to speed up the process.
"To avoid undue delay in giving customers important information we decided to err on the side of caution so that all affected customers would receive notification," said a spokesman for the company.
"We are writing to the customers told to boil water, but who are actually outside the Cwellyn supply area, to inform them that they no longer need to boil their drinking water."
There are now two helplines in operation for people with concerns to contact.
Around 70,000 householders supplied by the Cwellyn reservoir near Caernarfon have been asked to either boil their water or drink bottled supplies until 9 January at the earliest.
Welsh Water has said customers might receive compensation for the inconvenience.