Sierra Leone on Monday announced a new Ebola death in a northern district that had gone nearly six months without reporting any infections. The announcement marked another setback in ending Ebola transmission in Sierra Leone, which has totaled nearly 4,000 Ebola deaths during the worst outbreak ever recorded.

In August, Sierra Leone’s last known Ebola patient was released from a hospital after recovering, a milestone that allowed the West African nation to begin a 42-day countdown toward being declared free of Ebola transmission. But less than a week later, officials confirmed that a 67-year-old woman had died of Ebola in the northern Kambia district.

The latest fatality is a 16-year-old who died on Sunday in Bombali district, about 150 miles northeast of the capital, Freetown, according to a statement from the National Ebola Response Center issued Monday.

A swab from her body tested positive for Ebola, but the source of the infection is unknown, the statement said.

The village where she died has been placed under quarantine. Monday’s statement expressed confidence that Ebola could be contained in the district. “Bombali District has not reported a case for 169 days, however, this does not mean it is not ready to respond,” it said.

Officials said last week there were four Ebola patients receiving treatment in Sierra Leone, all of whom were believed to be relatives of the woman who died in Kambia district. A niece of the woman who officials think may be at risk of Ebola has not been located.