Liberian health authorities are taking rapid precautionary steps after eight people died of a mystery illness, the World Health Organization said on Wednesday, 10 months after the end of a catastrophic two-year Ebola virus outbreak.

Liberian authorities said initial scientific investigations ruled out Ebola as the cause of the deaths.

“Yesterday WHO received a report from Liberia health authorities about a cluster of unexplained illness and deaths in the southern part of the country – Sinoe County,” WHO spokesman Tarik Jasarevic said in emailed comments.

“According to this report, since Monday 14 people have fallen sick. Eight people have died and six are seriously ill and still in the hospital.”

He gave no indication of what might have caused the deaths but said specimens from seven bodies had been sent to the national laboratory for testing. Results are expected later on Wednesday or Thursday.

Samples were also being taken from water sources to test for chemicals and bacteria.

“Health authorities are taking immediate precautionary measures such as isolating suspect cases, tracing contacts and engaging with the community and their leaders,” Jasarevic said.

Rapid response teams have been activated at district and county level, he said.

The Chief Medical Officer of Liberia, Dr. Francis Kateh, told a news conference in Monrovia on Wednesday:

“Initial test conducted by the Liberian Institute of Biomedical Research in Charlesville, Margibi County, has ruled out Ebola virus disease as the cause of the deaths.”

In June last year, the WHO declared Liberia free of active Ebola virus transmission, the last of three West African countries at the epicenter of the world’s worst outbreak of the disease.

The epidemic killed more than 11,300 people and infected some 28,600 from 2013, as it swept through Guinea, Sierra Leone and Liberia, according to WHO data.

(Reporting by Tom Miles; additional reporting by James Harding Giahyue in Monrovia)