The World Health Organization (WHO) says it is extremely worried about a cluster of recent human deaths from the virulent H5N1 strain of bird flu.
Indonesia's bird flu death toll is second only to that of Vietnam
Seven people from the same family in northern Sumatra, Indonesia, died from the disease earlier this month.
WHO spokesman Peter Cordingley said there was no sign of diseased poultry in the immediate area.
Investigators are looking into the possibility that the virus spread from human to human, Mr Cordingley said.
But he emphasised that there was no indication the virus had mutated.
Experts are worried that if it does mutate, the H5N1 strain could become more easily transmitted between humans, leading to a worldwide pandemic of the killer disease.
The H5N1 virus has already killed more than 120 people worldwide since 2003. It has also devastated poultry stocks.
The majority of deaths have occurred in Asia, but cases in people and birds have also been recorded in Europe and Africa.
Almost all human infections so far are thought to have been caused by direct contact with sick poultry.
Mr Cordingley said that the Sumatran cases presented a major puzzle, as they were the largest cluster of human cases to date.
"[This] is probably the most worrying incident so far since bird flu started nearly three years ago and we can't find any obvious source of infection. We can find no sign of infected chickens; no sign of the virus in the environment around where they live," he said.
All seven people who died were members of the same family. An eighth family member is also thought to have the disease.
So far investigators know that the initial victim was a woman, who became ill at the end of April. She died in early May and was buried before laboratory tests could be carried out.
The subsequent six victims - all of whom were positively identified as having the virus - had close and prolonged exposure to either her or other family members with the disease, the WHO said.
Clusters of bird flu cases are viewed with far more concern than isolated infections, because of the possibility of transmission between humans.
There have already been several cluster cases - such as one seen in Thailand in 2004 - although they have always involved fewer individuals than the Indonesian case.
But the possibility scientists are most worried about, genetic mutation of the virus, has been effectively ruled out in this case.
"Sequencing of all eight gene segments found no evidence of genetic reassortment with human or pig influenza viruses, and no evidence of significant mutations," the WHO said in a statement on Wednesday.
The WHO is continuing its investigations into the Sumatran case.
But the organisation says there is so much grief in the village that it is difficult for officials to get enough co-operation from local people to do their job.