A Chinese man who died of pneumonia in 2003 and was at first classified as a SARS victim might have in fact died of avian influenza, Chinese researchers reported on Wednesday.

But in a confusing development, at least one of the researchers asked that the letter reporting the case be withdrawn from publication in the New England Journal of Medicine. Editors of the medical journal said they were trying to find out why.

The letter was available to journalists before its withdrawal, and describes the case of a 24-year-old man who died from pneumonia and respiratory distress in November 2003.

"Because the clinical manifestations were consistent with those of the severe acute respiratory syndrome (SARS) and occurred when sporadic cases of SARS were described in southern China, serum and lung tissue from the patient, as well as fluid aspirated from his chest, were examined for SARS coronavirus," the researchers wrote. "All tests were negative for SARS."

The World Health Organization said it was asking China´s Ministry of Health for clarification.

"This has been signed by eight scientists from very prestigious institutions. It certainly adds weight to the information," said Roy Wadia, WHO spokesman in China.

SARS first broke out in China´s southern Guangdong province in 2002 and spread as far afield as Canada before it was brought under control in 2003. It killed close to 800 people out of the 8,000 known to have been infected.

Ironically, experts at the time assumed the then-mysterious illness making people sick in China was H5N1 avian influenza, which broke out in Hong Kong in 1997 and then disappeared.

Wadia said: "It reinforces what we have known for a very long time which is that the H5N1 virus has been in the environment of this part of the world for a while and it´s therefore not surprising that you would have these sort of cases."

"This (case) was the same time that the virus had started taking human lives in some other countries, Vietnam, Thailand."

"In fact, WHO said in February 2004, when China was awash in poultry outbreaks at the time, that it would not be inconceivable that there could be sporadic human cases on the Chinese mainland that may not have been tracked or confirmed."

Influenza experts say flu viruses rarely just disappear and had been waiting for its return, which was reported in 2003.

In the case of the Chinese man, tests of his tissue were positive for influenza virus and genetic sequencing later showed it to be H5N1 avian influenza.

It genetically resembled samples of viruses taken from Chinese chickens in various provinces in 2004, the letter says. Parts of the virus also resembled Japanese samples.

The eight researchers who signed the letter include Dr. Wu-Chun Cao of the State Key Laboratory of Pathogens and Biosecurity, Dr. Qing-Yu Zhu of the State Key Laboratory of the Beijing Institute of Microbiology and Epidemiology, and Dr. Wei Wang of the 309th Hospital of the People´s Liberation Army.

They said the virus infecting the man had mixed lineages and that their findings were important for developing an eventual bird flu vaccine.

"The genetic distance between the isolate reported and the strain currently proposed for vaccine development implies that viruses from different regions may need to be considered in the development of an effective vaccine against influenza A virus," they wrote.

The H5N1 avian flu virus has swept across most of Asia, into parts of Europe and Africa. While it is mainly a disease of birds, it occasionally infects people and has killed 130.