World Health Organisation experts widened their probe of China's SARS cases today, interviewing people at the hospital where a lab worker was treated after getting the disease on the job.
So far, China's new cases of the highly contagious ailment are limited to people who worked at Beijing's Institute of Virology where SARS samples are kept and others who came in contact with them.
China has reported five confirmed cases and four suspected ones. One of the confirmed SARS patients has died.
WHO blames lab security for the mini-outbreak but has yet to nail down exactly what happened.
"It still remains very uncertain, the source of this infection," said Dr. Julie Hall, SARS team leader for WHO in Beijing. "This is going to take some time."
She said WHO experts visited the SARS lab on Friday. On Saturday, they went to the hospital where a sick lab worker was treated and where a nurse has since come down with the disease.
Ms Hall said WHO's visit to the lab produced few clues. The lab and its equipment are new, she said, and there were no obvious spills reported. Experts will need to interview more researchers at the lab to learn their day-to-day practices and figure out what went wrong, she said.
Meanwhile, millions of Chinese boarding planes and trains for the weeklong May Day holiday were being screened for SARS symptoms to ensure the disease remained contained.
China confirmed yesterday that a 53-year-old woman who died last week was the world's first knwon SARS death this year. Her daughter worked at the SARS lab and is also a confirmed case.
The other confirmed cases are a nurse who treated the sick lab worker and the nurse's mother and aunt.
Hundreds of people who had contact with them and the suspected SARS patients are isolated and under medical observation.
SARS, or severe acute respiratory syndrome, sparked a global health crisis last year when it killed 774 people worldwide and infected thousands.