World Health Organization officials said Friday that they had identified a new case of Ebola in Liberia in a 10-year-old boy who lives close to the country's capital, Monrovia.

The boy became sick on Nov. 14 and sought medical help. His infection with the deadly virus was confirmed, but health investigators have not been able to determine how he was exposed. He has no known history of contact with a survivor, travel to an affected area or other factors they typically look for to explain a case.

"We don't have a lot of information because investigation is ongoing," Bruce Aylward, the World Health Organization's special representative for the Ebola response, said. "As the investigation goes forward, I'm sure we will start to see new clarity."

Aylward said the boy's family members are "at particular risk" and are being monitored. Two of his siblings have been "unwell" in recent days, he added.

Aylward said it's possible the boy's case is not part of the original outbreak - which appears to be over - but may be due to the persistence of the virus in some survivors who can then pass it along to others. Recent studies have shown that the virus can live in the semen, eyes, and other parts of the body long after it appears to be cleared in the blood.

He said this reflects "a fundamental shift in the epidemiology of the outbreak."

Since the spring of 2014, Liberia has suffered more than 10,600 cases and seen 4,808 deaths.