Medical samples from four of the victims of a disease in Liberia that initially baffled scientists have tested positive for a type of meningitis, the minister of health said on Monday.

So far a total of 31 cases of the so-called mystery illness have been reported, including 13 deaths in an outbreak linked to the attendance of a religious leader’s funeral.

While the symptoms are different from Ebola, the sudden deaths nevertheless stirred anxiety about an outbreak of the tropical illness which was often spread through burial rituals in the West African epidemic that ended less than a year ago.

Seven specimens from the deceased tested positive for Neisseria meningitis, a particularly contagious type of bacteria, Dr. Bernice Dahn told reporters on Monday.

“Based on these initial results from the CDC Atlanta (U.S. Center for Disease Control), we believe that we are dealing with a probable outbreak of meningitis in Sinoe, which spread to Montserrado and Grand Bassa counties,” she added, while calling for calm.

She said the government was investigating options for mass vaccinations against the illness, characterised by swelling of the membranes in the brain and spinal cord. Further laboratory analysis is