New Zealand officials are investigating the possibility that its first Covid-19 cases in more than three months were imported by freight, as the country’s biggest city plunged back into lockdown on Wednesday, August 12, Reuters reports.

The discovery of four infected family members in Auckland led Prime Minister Jacinda Ardern to swiftly reimpose tight restrictions in the city and social distancing measures across the entire country.

The source of the outbreak has baffled health officials, who said they were confident there was no local transmission of the virus in New Zealand for 102 days.

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

Investigations were zeroing in on the potential the virus was imported by freight. Bloomfield said surface testing was underway at an Auckland cool store where a man from the infected family worked.

“We know the virus can survive within refrigerated environments for quite some time,” Bloomfield said during a televised media conference.

China has reported several instances of the coronavirus being detected on the packaging of imported frozen seafood in recent weeks.

The World Health Organization website states there is currently no confirmed case of Covid-19 transmitted through food or food packaging. However, it also notes that studies have shown that the virus can survive for up to 72 hours on plastic.