The source of a new outbreak of Ebola in Liberia remains a mystery, experts said on Wednesday, describing the cluster as "separate" from the epidemic that killed thousands.
The west African nation announced last week that a 17-year-old boy had died of the tropical fever after spreading it to two other people, in the first cases of infection for more than three months.
"The origin of infection of the cluster of cases is currently under investigation," the World Health Organization (WHO) said in the latest of its weekly reports on the progress of the epidemic.
"At present, these cases are considered to constitute a separate outbreak from that which was declared over on May 9."
The epidemic killed more than 4,800 Liberians before the WHO declared the country free of transmission, 42 days after the last confirmed case was buried.
Liberia's neighbours Guinea and Sierra Leone are both battling the outbreak, which has killed more than 11,200 people, but the coastal Margibi County, where the new cluster was uncovered, is nowhere near either border.
The 17-year-old in this case is not thought to have travelled to either neighbouring country, but was shown to have had contact with the two other confirmed cases, who are receiving emergency treatment in Monrovia.
A fourth case who had contact with the boy is being treated as "probable Ebola" after developing symptoms, according to the WHO, although infection has not yet been confirmed.
The virus is spread among humans via the bodily fluids of recently deceased victims and people showing symptoms of the tropical fever, which include vomiting, diarrhoea and -- in the worst cases -- massive internal haemorrhaging and external bleeding.
Experts have speculated that the teenager could have been infected by an entirely new variant of the virus from an animal such as a fruit bat, rather than from a human.
A more worrying possibility is that clusters of Ebola continue to smoulder under the surface, far from the gaze of local or international health authorities.
Rubein Duo, the tribal chief of the area of the new outbreak, said the victim had cooked and eaten a recently deceased dog with his friends shortly before falling ill.
There is limited evidence that dogs can become infected with Ebola but none has ever developed disease or demonstrated the ability to pass the virus to humans, however.
"Though it is true that they ate the dog, that is not where they got the virus. We are exploiting other possible routes, and when confirmed we will inform the public on how they got the virus," Cestus Tarpeh, a spokesman for the health authorities in Margibi County told AFP.
There were 30 confirmed cases reported in the week to Sunday -- 18 in Guinea, three in Liberia, and nine in Sierra Leone -- the highest weekly total since mid-May but a fraction of the numbers during the epidemic's peak last year.