Seven out of eight members of the same family who contracted bird flu have died from the disease in what the World Health Organisation describes as the "most worrying incident so far".
It is the largest cluster of human cases of infection with H5N1 since the outbreak began three years ago and scientists have been unable to identify the source.
The seven relatives in northern Sumatra, Indonesia, died earlier this month but extensive tests have failed to turn up any sign of diseased poultry in the area. Scientists are left facing the possibility that the virus passed among the family by human-to-human transmission.
However, analysis of samples taken from the family have not shown any mutation which could signal the start of a human pandemic, the WHO said.
Peter Cordingley, spokesman for the Western Pacific region of the WHO, said: "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." Scientists were "completely stumped", he added.
The WHO said it had considered raising its pandemic alert level but decided that the situation was not serious enough to warrant it.
The outbreak, in the village of Kubu Sembelang, began in late April when a woman fell ill with a respiratory disease and died on 4 May. Although she was buried before tests could confirm if she was infected with H5N1, she spent the night of 29 April, coughing frequently, in a small room with three relatives who also became infected.
It is thought that they may have passed the virus to other family members. The 10-year-old son of one of the woman's brothers died of bird flu on 13 May. The boy's father developed symptoms on 15 May and died on 22 May.
A spokesman for the WHO said previous human clusters of bird flu had been seen suggesting human-to-human transmission but the chain of transmission had always ended with the second person to be infected.
"That seems likely in this case. We haven't seen health workers infected [who cared for the family]. If they were that would seriously raise our concerns."
Viruses isolated from the family were flown to WHO reference laboratories in Hong Kong and the United States for analysis but no significant mutation was found. "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.
Bird flu has killed 124 people worldwide, more than a quarter of them in Indonesia, since the virus started ravaging poultry stocks across Asia in 2003. So far, most human cases have been traced to contact with infected poultry.
Firdosi Mehta, acting representative of the WHO in Indonesia, urged against any over-reaction, saying that there was no sign of the virus having spread among villagers. She said: "We are putting [anti-viral] Tamiflu on whoever has had close contact, basically putting family members who have not been affected on Tamiflu as a precaution.
"There is active surveillance in the village to look for any more cases that are occurring outside this family cluster."
Financial markets, however, reacted to fears that the Indonesia cluster could be the start of a pandemic. Currencies in Asia fell, US commodity prices came under pressure while markets around Europe also slipped.