Bats may be the source of a new Sars-like virus which killed a man in Saudi Arabia, according to an analysis of the coronavirus' genome.
Two other people have been infected and one, who was flown to the UK for treatment in September, is still in intensive care.
Experts, writing in the journal mBio, said the virus was closely related to other viruses in bats.
It is thought the virus does not pass readily from one person to another.
Coronaviruses are a family of viruses ranging from the common cold to the Sars (severe acute respiratory syndrome) virus. They infect a wide range of animals.
In 2002 an outbreak of the Sars coronavirus killed about 800 people after it spread from Hong Kong to more than 30 countries around the world.
The new coronavirus was detected after a 60-year-old man died of pneumonia and kidney failure in Saudi Arabia in June.
A second man, who was brought to the UK from Qatar, still needs an artificial lung to stay alive.
The UK's Health Protection Agency (HPA) published the full genetic sequence of the case in London earlier this month.
Dr Ron Fouchier, from the Erasmus Medical Centre in the Netherlands, analysed the case from Saudi Arabia.
He said both cases were related, but it looked as though the men had been infected separately from animals rather than the virus being passed between people.
"The virus is most closely related to viruses in bats found in Asia, and there are no human viruses closely related to it therefore, we speculate that it comes from an animal source."
Prof Maria Zambon, the director of the HPA's reference microbiology services, said bats were a natural reservoir of coronavirus: "There's some in cats and in humans, but the broadest range is found in bats."
However, she said it was not "definitively" from bats as the virus could have spread to another animal first, which then passed the virus onto humans.
Researchers are trying to determine if the virus will be a "dead end" infection which can spread only from animal to person, like rabies, or will be able to spread from one person to another like HIV after it made the jump from primates.
Prof Zambon said there were "fairly strong steers" that it would be a dead end. Health care workers who came into contact with the patients "don't seem to be ill" but full blood test results are still needed, she said.