Authorities in New Zealand are investigating the possibility that its first coronavirus cases in more than three months were imported by freight, as the country’s biggest city plunged back into lockdown on Wednesday.

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

Health officials said they were confident there was no local transmission of the virus in New Zealand for 102 days. Investigations were zeroing in on the potential the virus was imported by freight, Reuters reports. Testing was underway at an Auckland cool store where a man from the infected family worked.

Residents of Auckland, home to around 1.7 million people, were given hours to prepare for the return to level 3 restrictions on Wednesday, requiring people to stay at home unless for essential trips.