Argentina is trying to solve a medical mystery after 57 sailors were infected with the coronavirus after 35 days at sea, despite the entire crew testing negative before leaving port, AFP reported.

The Echizen Maru fishing trawler returned to port after some of its crew began exhibiting symptoms typical of Covid-19. According to the health ministry, 57 sailors, out of 61 crew members, were diagnosed with the virus after undergoing a new test.

However, all of the crew members had undergone 14 days of mandatory quarantine at a hotel in the city of Ushuaia. Prior to that, they had negative results, the ministry said in a statement.

“It's hard to establish how this crew was infected, considering that for 35 days, they had no contact with dry land and that supplies were only brought in from the port of Ushuaia,” said Alejandra Alfaro, the director of primary health care in Tierra del Fuego.