MONROVIA (Liberia) • Three new cases of Ebola have emerged in Liberia since the country was declared free of the virus on May 9, highlighting the continuing threat of viral outbreaks in West Africa.

The Liberian government said it was tracing scores of people who had recent contact with the three patients,combingthe villagewhere the first of the infected people died, and isolatingmore possible cases in an Ebola treatment centre.

Officials have yet to determine the source of the fresh infections, whether they are related, or whether they involve a new strain of the virus or a re-emergence of a knownstrain.

"We're looking at all possibilities," said Dr Thomas Frieden, director of the United States Centres for Disease Control and Prevention (CDC), who said his agency was assisting the Liberian government with its response.

The government of President Ellen Johnson Sirleaf took several precautionary steps on Thursday to help head off any further spread of the virus.

The Health Ministry said it expected to have thermal cameras set up at the main international airport by the end of the day, to spot travellerswhomaybe running a fever.

The Education Ministry said it would close schools in the country until September, when they would reopen in a restoration of the country's usual academic calendar after the disruptions of the past year, when Ebola killed nearly 5,000 people in Liberia alone.

The first of the three new cases was not identified until the patient, a 17-year-old boy from Margibi County, had died. Tests of samples taken from his body confirmed on Sunday that he was infected with the virus.

TheLiberian government is treating the second and third cases as confirmed, but Dr Frieden of the CDCsaid that laboratory results for those two cases were indeterminate or only very weakly positive for Ebola.

"At this point, we are not certain that there are additional cases," he said. "We have had some challengeson the laboratory work."

NEWYORKTIMES