The mystery of how China’s latest SARS outbreak leaked from a national laboratory remains unsolved and it is too soon to establish if the infection which has killed one woman has ended, the United Nations said on Friday.

The World Health Organization (WHO) suspects all nine reported cases of the deadly flu-like virus are linked to the Chinese National Institute of Virology, which had been conducting experiments using the live SARS coronavirus.

But two researchers at the institute who caught SARS were not known to have come in contact with live SARS samples, only inactive ones, the WHO said.

“So there is no smoking gun, if you will, at this time,” Beijing-based WHO spokesman Roy Wadia said.

The mother of one of the researchers, a medical student from the eastern province of Anhui, is the only person to have died from the latest outbreak of severe acute respiratory syndrome.

The first case to be detected, a nurse who treated the student in Beijing, became the first to leave the hospital on Wednesday. The only patient still in serious condition is a 49-year-old man who shared a hospital room with the nurse, according to the health ministry.

“At this point we haven’t heard of any new cases and so if the trend continues over the next several days and weeks, then it would certainly seem to be encouraging,” said Wadia.

“But it’s too early in our estimation to say that the outbreak is under control,” he added. “We’re not using that phrase yet.”

WHO biosafety experts have made two visits to the institute and were scheduled to visit again within days, Wadia said. They also interviewed the female medical student but have yet to speak to another SARS patient who worked alongside her in the lab because he was too ill, he said.

SARS first emerged in southern China in late 2002 and was spread across the world by travelers. It killed nearly 800 people in various countries before the outbreak was declared over in July last year.

More than 300 people isolated for observation in the past couple of weeks have been released, but hundreds of others were still quarantined, the health ministry said.