A Liberian boy died of Ebola on Monday, in the first death from the disease since the country was declared Ebola-free in September.

The 15-year-old died Monday night in a hospital near the capital after testing positive for the disease last week, the Associated Press reported from Monrovia.

His father and brother are being treated for Ebola, too.

The September declaration was the second time it received such a designation this year. Liberia was also deemed free of the disease in May, but new cases surfaced a month later.

In this case, nearly 160 people risk being infected by the disease, the Associated Press reported. At least eight of them are health-care workers at "high risk" due to having direct contact with the boy.

The country enlisted the help of two Centers for Disease Control and Prevention experts to investigate the case.

"Our working hypothesis is that the virus is reintroduced into the human population through uninfected people, and we know that it is a possibility that people who have been infected with the virus previously may continue to transmit," Liberia's representative to the U.N. World Health Organization, Dr. Alex Gasasira, said, according to Reuters.

The virus can survive in semen, the eyes, and other parts of the body long after it appears to be out of the bloodstream, recent studies have found.

Officials first identified a new case of Ebola in the country last week in a 10-year-old boy who became sick Nov. 14.

The virus has claimed more than 11,300 lives since it broke out in West Africa in March 2014, according to WHO figures.

The outbreak was concentrated in three countries. Liberia was home to roughly 42 percent of the Ebola deaths, while Sierra Leone and Guinea had 35 percent and 22 percent shares of the overall deaths, respectively.