A karaoke party at a house on the outskirts of Limavady is understood to be the cause of a Covid-19 outbreak.
It is believed partygoers followed social distancing guidelines, but it is thought the virus was transmitted by the microphone being passed around.
Health officials are now conducting a test and trace programme to establish how many other people have the virus.
One of the people being tested on Thursday is understood to work in a shop in the area, raising concerns that customers may unwittingly have been infected and passed the virus on.
It comes after the Public Health Agency (PHA) said it has identified a cluster of Covid-19 cases in the Limavady area as the Department of Health revealed 16 people were diagnosed in the most recent 24-hour period.
Nine people were diagnosed across Northern Ireland the day before.
In total, there have been 40 new cases in the last seven days.
There have been 14 new positive cases in the Causeway and Glens area in the past seven days, according to Department of Health figures.
It is not known exactly how many of these are linked to the karaoke party outbreak.
A health service source said the outbreak is not unexpected in light of the relaxation in social distancing measures; however, it is hoped the contact tracing programme will help to limit the spread of the deadly virus.
Health Minister Robin Swann said: "We have long anticipated and warned that clusters will happen.
"I want to commend colleagues in the PHA for the work they are doing in keeping people safe through the Test, Trace and Protect programme.
"I would once again urge everyone across Northern Ireland to guard against complacency.
We have made important strides forward against the virus, but we must always remember that we have a long way to go. We face a very uncertain next few months and the very real prospect of a very challenging autumn and winterDr Michael McBride
"The virus is still present in our community and must never be underestimated.
"Please stick to social distancing at all times and keep washing your hands regularly with the same rigour as at the start of this pandemic."
Chief Medical Officer Dr Michael McBride said: "Today's news underlines once again that we have to keep up our guard against Covid-19.
"Social gatherings in private residences have been an issue on both sides of the border.
"We have made important strides forward against the virus, but we must always remember that we have a long way to go. We face a very uncertain next few months and the very real prospect of a very challenging autumn and winter.
"Please keep following the public health advice at all times. Stay safe and save lives. And make sure you avoid crowds, closed spaces and close contact when you are socialising."
The PHA said: "At present, we can link the cases to a social gathering in a private or residential area.
"Testing and contact tracing is being conducted to identify people who may be potentially affected to help prevent any further spread.
"At present anyone who tests positive for Covid-19 in Northern Ireland will be contacted by the contact tracing team to be given advice on what to do about managing symptoms and of the need to self-isolate to prevent any wider spread of the virus.
"They will also be asked who they have been in contact with so that we can follow up with those individuals to give them advice to help prevent the further spread of infection."
The PHA warned against identification of anyone who has been diagnosed as part of a cluster as there are concerns this will discourage people from coming forward to be tested.
No Covid-19-related deaths were recorded by the Department of Health here in the previous 24 hours, and the toll from the disease remains at 556.