Scientists had thought Sars was a new type of coronavirus
Canadian doctors dealing with Sars have cast doubt on the origins of the virus that has killed more than 350 people around the world.
Speaking at the Sars conference being held in Toronto, they say they find coronavirus - believed to cause the disease - in under half of their Sars patients.
China, the worst affected country, is struggling to cope with the disease, with the mayor of Beijing saying that hospitals cannot keep pace with the spread of Sars.
And in China's Inner Mongolia region, a new 1,000 bed hospital is to be built from scratch in 20 days to accommodate Sars patients.
China reported 11 more Sars deaths and 166 new cases on Wednesday, bringing its total deaths from the virus to 159 and the case load to 3,460.
Inner Mongolia has seen nine deaths, 127
cases and 220 suspected cases.
While new cases appear to be tailing off in Hong Kong and other affected areas, they are still on the increase in mainland China.
Known death tolls:
Mainland China: 348
Hong Kong: 298
Source: WHO/local authorities
"The situation in Beijing remains severe for Sars prevention and treatment. Infections have not yet been cut off," the city's acting mayor, Wang Qishan, said in a statement.
Mr Wang, who was appointed last week after his predecessor was fired for mishandling the outbreak, said Beijing has identified 21 hospitals to tackle the disease.
But he added: "Due to a shortage of berths at designated hospitals, not all suspected Sars patients can be hospitalised there in a timely manner."
The BBC's Richard Black says that two weeks ago it seemed that the cause of Sars had been firmly identified.
Laboratories in Hong Kong and the United States isolated a new strain of coronavirus from patients.
Researchers in Germany then infected monkeys with this virus and found they developed all the symptoms of Sars, which satisfied the World Health Organization that the virus caused the condition.
But now Canadian doctors says only 40% of Sars patients seem to be infected with coronavirus and some people are infected but do not have Sars.
If the current policy of isolating and quarantining patients isn't enough to contain Sars. drugs and vaccines will be needed - and for that scientists will need to know precisely which virus they're developing drugs and vaccines for, our correspondent says.
An American researcher has suggested a different possible origin for Sars: military biological weapons programmes.
Richard Fisher, a senior fellow at the Jamestown Foundation, a Washington-based think tank, said the theory shouldn't be ruled out.
Other China-watchers are more sceptical however, pointing to a lack of any hard evidence.