TORONTO, May 27 -- Canadian officials investigating a new cluster of SARS cases admitted today they were too quick to let down their guard against the resurgent virus, which they thought had been controlled.

Thirteen days after Toronto was declared virtually SARS-free, officials said they were treating nine probable and 23 suspected cases of the respiratory ailment in a new outbreak they labeled SARS II.

More than 3,400 people have been asked to enter quarantine, including 185 health care workers, officials said. "This is another testament to how effective this virus is in doing its job, transmitting from person to person," said Donald Low, chief microbiologist at Mount Sinai Hospital in Toronto. "The only good news is, it is contained to hospital settings."

Canadian health officials and representatives of the U.S. Centers for Disease Control and Prevention were sent to Toronto to investigate the new cases. Ontario health officials said they were still investigating what caused the new outbreak. So far, they have tracked the new cases to a 96-year-old man who developed pneumonia and died after surgery in the orthopedic unit of North York General Hospital. Health care workers assumed the man was suffering only from symptoms of pneumonia, which is a common infection in elderly patients following surgery. Officials suspected the man had SARS only after health care workers in his hospital unit developed symptoms.

Ontario officials said they were researching files to ascertain how the man was infected, although they believed he was linked to the original cluster of cases from Scarborough Grace Hospital, the epicenter of Toronto's SARS outbreak in March.

The elderly patient did not display classic symptoms of SARS, or severe acute respiratory syndrome, said Colin D'Cunha, Ontario commissioner of public health. "A couple of people got infected at North York General." The web of the virus then spread to a third and fourth hospital as more patients were transferred.

"One took it to St. John's rehab where he shared a room with someone else who came down with SARS," D'Cunha said. Another patient in the orthopedic unit contracted the virus. That patient was transferred to St. John's Rehabilitation Hospital where he transmitted the disease to a health care worker and another patient.

Jeffrey Lozon, president and CEO of St. Michael's Hospital, said the disease spread to his facility when two patients were transferred there. One of those patients was admitted to the neurosurgery unit and the other was treated in the coronary care unit. On Monday, St. Michael's closed both units. "The reason we stood down the neurosurgical program is 80 percent of our direct care staff have been quarantined, with the exception of one who was on vacation," Lozon said. About 50 staff workers in the coronary care unit have been quarantined.

D'Cunha said all hospitals in the greater Toronto area have been asked to screen all patients. He said hospitals would operate under a "new normal" of heightened vigilance and increased protection "as long as there are SARS patients available in the world."

"We shouldn't forget the lessons we learned in March and April," said D'Cunha.

Tony Clement, Ontario's health minister, said, "We live in a state of continued vigilance. We can never let our guard down."

Taiwan officials reported 14 probable new cases of the virus and said five more people had died. The total number of SARS cases in Taiwan rose to 610 from 596, the third-highest in the world after mainland China and Hong Kong. The death toll in Taiwan climbed to 81.