Officials investigating an outbreak of mild bird flu are testing birds on a second farm in north Wales.
The outbreak was confirmed at the Conwy smallholding on Thursday
Nine people were tested after the H7N2 strain was found in chickens in the first case at a smallholding in Conwy, with four people testing positive.
The poultry had been bought from the Chelford Market near Macclesfield.
The second possible case is about 35 miles (56km) away on the Llyn Peninsula, Gwynedd, with birds being tested because of links to the market.
Chief veterinary officer Dr Christianne Glossop said there was a police presence at the Gwynedd farm to ensure no "unnecessary access" to the premises. The farm is understood to be on the outskirts of Efailnewydd, near Pwllheli.
She said: "We are testing birds at the farm because of its link with Chelford Market on 7 May 2007.
"A decision on any further action required regarding birds on the farm will be taken on the basis of the test results."
Health officials have stressed that the disease found at the farm in Conwy was the H7N2 strain of bird flu, not the more virulent H5N1.
The owners of the Conwy smallholding at Llanfihangel Glyn Myfyr, near Cerrigydrudion, Tony Williams and Barbara Cowling, have tested negative for the virus.
The 15 birds found with the virus were bought at Chelford Market
The couple called in a vet after the Rhode Island Red birds, which they bought at the market about two weeks ago, had started to die.
Samples from the birds were sent for testing, and the virus was confirmed on Thursday.
Earlier on Saturday, Dr Glossop confirmed the birds had been bought at the market, which is some 70 miles (112km) away from the Conwy farm, and said it was being regarded as one line of inquiry.
She said: "An important part of disease control is finding the source of infection.
"It has to be a very thorough process with a number of lines of inquiry to follow."
Announcing the latest tests of birds on the Llyn Peninsula, she reiterated her appeal for anyone who purchased from or supplied to the market on Monday, 7 May and had not already been contacted by animal health officials, to contact their local office.
Similarly, any poultry keeper who visited the market on that day and whose birds have subsequently become ill should do so, Dr Glossop said.
"In the meantime, all bird keepers throughout Wales should continue their efforts to maintain high levels of biosecurity and maintain vigilance by continuing to monitor their birds for signs of disease," she continued.
"If you are concerned about the health of your birds you should seek the advice of your veterinary surgeon, if you suspect that your birds have avian influenza you should report it to your local animal health office."
Three of the nine people tested for the virus were taken to hospital, but have now been discharged.
Chief medical officer for Wales Tony Jewell said on Friday: "I would like to reassure the general public that the risk to their health from this outbreak is very low.
"This particular strain is not highly pathogenic and is normally only contracted following close contact with infected birds. In addition the symptoms are generally mild."
Two of the four people who have tested positive were from Wales and the other two were from north-west England.
The five people who tested negative are being treated as having had the virus as a precaution.
The Department for Environment, Food and Rural Affairs (Defra) wants to hear from anyone - who has not already been contacted - who purchased from or supplied to Chelford Market, Cheshire on Monday, 7 May, or any poultry keeper who visited the market on that day whose birds have subsequently become ill.
They are asked to contact their local animal health office or the Defra helpline 08459 335577. Lines are open between 0900 BST - 1700 BST seven days a week.