Following a 102-day run without any local transmission of COVID-19, the country was placed back under lockdown Wednesday after four members of a family in Auckland were found to have contracted the virus.

“We are working hard to put together pieces of the puzzle on how this family got infected,” New Zealand’s Director General of Health Ashley Bloomfield said.

The island state had declared itself virus-free in June, with no active cases across the country. The government lifted all local lockdown restrictions, allowing people to meet and mingle at ease while maintaining strict quarantine for inbound travelers.

With no prior local cases and little chance of an infected person arriving in New Zealand, the country is now investigating whether the virus could have been imported through freight—specifically, refrigerated supply lines.

“We know the virus can survive within refrigerated environments for quite some time,” Bloomfield told a media conference. In Auckland, authorities are testing a cold storage unit, where one of the infected family members worked, for the virus.

The city—the largest city in New Zealand—is under a level 3 lockdown, requiring people to stay home except for essential trips, while officials attempt to trace the virus.

Members of the infected family had traveled to a local tourist site and another member had gone to work, where other staff are now showing symptoms.

The rest of the country is under a looser lockdown protocol, which will be in place until at least Friday.

Other countries have speculated previously that refrigerated supply lines helped transport coronavirus, too. In June, Beijing deployed “wartime” lockdown measures after a COVID-19 outbreak was traced to a seafood market; the virus was suspected to have been imported along with salmon.

In July, China’s coastal city of Dalian reported a new COVID-19 cluster, which it suspected was imported via frozen fish. And this week another Chinese city, Yantai, said it had found COVID-19 on imported packages of frozen seafood.

It’s important to note the virus isn’t thought to be transmitted by frozen food itself. Rather the virus can survive on the surface of shipped packages and products and infect people when the cargo arrives. Seafood has emerged as a potential risk because, according to China’s chief epidemiologist, “seafood products are typically stored and transported in cold containers, thus it is possible for the virus to be preserved for a long time” in a dormant state.

As an additional response to the outbreak, New Zealand Prime Minister Jacinda Ardern delayed dissolving Parliament—a political process that needs to be done before the country can hold its next general election, due in September.

“We’re seeking advice around the options of the issue of the election from the electoral commission, just so that we make sure we have all those options available to us,” Ardern said.