The deadly Ebola has re-emerged in Liberia, two months after it was declared free of the disease.

According to Los Angeles Times, a 15-year-old boy recently contracted the virus, and his father and brother also tested positive for Ebola on Friday.

Liberian health authorities have placed 153 people under monitoring, including 22 nurses and six patients from a hospital in the country’s capital, Monrovia.

CNN quoted Liberia health minister Dr. Bernice Dahn as saying, “It is possible that we will find more cases. The key is to stop it, find the source, and prevent the next one.”

It is unclear how the 15-year-old teen contracted Ebola but Liberian Health Minister Francis Kateh said investigation will continue to determine how the virus was transmitted.

Kateh said Liberia requested the assistance of the United States after three new cases were reported. At least two officials from the US Center for Disease Control and Prevention will travel to Liberia to help determine the cause of the resurgence of the disease, he added.

The West African country has been declared Ebola free twice this year. First in May but soon after a 17-year-old boy and a woman died from the disease in July. In September, the World Health Organization once again declared the country to be Ebola free after the maximum 42 days incubation period.

According to Reuters Africa, the previous resurgence of Ebola in Liberia is thought to have been via sexual transmission.The Ebola virus stays in the semen of male survivors for at least nine months after infection, which is much longer than its incubation period in blood.

Liberia recorded the highest death toll in the worst Ebola outbreak in history. The country suffered 4,808 deaths since the outbreak began in December 2013.

According to the latest World Health Organization data, Ebola had killed more than 11,300 people, mostly from Liberia, Sierra Leone, and Guinea. About 28,600 people were also infected of the virus.