The mass COVID-19 outbreak in the Cheonghae naval unit where 90 percent of its members were infected is believed to be traced to a country neighboring the Gulf of Guinea in Africa.

A joint investigation team including the military, medical sector and Korea Disease Control and Prevention Agency(KDCA) on Tuesday announced the results of an epidemiological survey into the outbreak.

The very first infection is presumed to have occurred some time between June 28 and July 1.

The spread is attributed to crowded conditions and the closed environment on the Navy destroyer, communal living, the highly transmissible delta variant, and a delay in recognizing symptoms due to confusion with the common cold and fatigue from long days out at sea.

The unit was flown home in late July to get tested and treated.

All 272 infected members of the 301-strong unit have now fully recovered and been released from quarantine. Of them, 64 were tested for the delta variant and found positive, indicating that the others also likely contracted the strain.