(CNN) An unexplained illness has claimed the lives of 12 people in Liberia since April 23, the World Health Organization reported Thursday. Twenty-eight people have fallen ill, including an unknown number of children, and three remain hospitalized at Francis Grant Hospital in Sinoe County.
The mystery began when an 11-year-old was admitted to a hospital April 23. The child had diarrhea and vomiting and was confused after attending the funeral of a religious leader the previous day, according to the WHO.
The child died within an hour of admission.
Symptoms of the illness include headache, diarrhea, vomiting and confusion, according to Fadela Chaib, a spokeswoman for the WHO. Health officials immediately tested the victims for Ebola, yellow fever and Lassa fever, but they were all ruled out.
All of the ill but one had attended a funeral April 22 in Greenville, Sinoe County. The exception was the partner of a man who had died in a hospital in the capital city, Monrovia. This partner became ill and died April 29.
"April 25, we got a call from the Sinoe County health team about a series of unexplained deaths," said Sorbor George, communication director for Liberia's Ministry of Health, adding that samples have been sent to the US Centers for Disease Control and Prevention in Atlanta.
According to Tom Skinner, a spokesman for the CDC, "timing of the events, duration between disease onset and death, higher case fatality among children are all suggestive of ingestion or exposure to a contaminant." Still, the CDC remains vigilant for other possible causes.
Several cases have tested positive for malaria and have been treated accordingly, Skinner said. He added that a CDC team is in Liberia and working closely with the government to provide additional support.
According to George, "We are still not aware of what killed them." Liberian health care providers are conducting new tests and are now "relying on the international community to help determine what the illness may be."
"We don't want to be specific as to the funeral being the reason for the death. We are still investigating and putting our acts together to know what happened," George said.
George said health officials should be receiving test results from the CDC in a week or two.
The ages of those who died could could not be confirmed.
People came to the funeral from six counties. Some of the victims died in the hospital in Sinoe County, a four-hour drive southeast of Monrovia. One person died in a hospital in Monrovia. Four people have been discharged from the hospital.
Those currently hospitalized have stabilized, and their status is improving, according to Tarik Jasarevic, a spokesman for the WHO, adding that "safe and dignified burial" has been conducted for eight of those who died.
Response measures include isolation and hospital staff wearing personal protective equipment, as during the Ebola crisis, Jasarevic explained.
A total of 42 individuals who participated in the burial are being closely monitored, as are contacts of those who became ill, according to the WHO.
Liberia, along with Guinea and Sierra Leone, had been at the epicenter of the global outbreak of that disease, which infected roughly 28,600 people and killed more than 11,300 in those three countries beginning in 2013. The WHO declared Liberia free of active Ebola transmission in June 2016.
Having taken samples from the dead, Chaib said scientists "will be looking of course for other hemorrhagic fevers and for bacteria, if there was any common exposure to water contamination or food contamination."
With an ongoing epidemiological investigation, health officials are continuing to search for active cases while tracing contacts, Jasarevic said. The WHO is also consulting with community leaders from the affected communities and mobilizing social networks to encourage people to go to the hospital if they feel sick.