TORONTO, May 29 -- More than 5,000 people were ordered into quarantine in Ontario today, and health officials reported 33 new probable cases and 29 suspected cases of SARS, raising fears the disease could spread rapidly once more.

Officials took measures to contain the new outbreak of the virus, which lingered unnoticed in hospitals until it was detected again last week. Authorities restricted hospital access and closed a liquor store after they learned that an employee may have been exposed to SARS during a hospital visit.

Medical officers issued warnings that those quarantined must obey the isolation order or face being locked in hospitals. Among 2,000 people previously quarantined are about 1,700 students from Father Michael McGivney Catholic Academy, which was closed after a student suffering from SARS symptoms attended classes last week.

"Everyone is having to adjust again as we are dealing with this whole new cluster of cases. We did the right things before. . . . We will have success again," said James Young, Ontario's commissioner of public security. He said the disease could be contained again if people wash their hands and stay in isolation and if medical investigators are successful in tracking SARS cases. "People have to abide by isolation whether they are teenagers or someone else," Young said.

Health officials, still recovering from an outbreak in March and April, said this second occurrence of SARS will worsen before the situation is controlled. They expect the number of cases to increase as the investigation continues and more people become critically ill.

The source of the latest outbreak is unknown, but officials are continuing to investigate how a 96-year-old man contracted SARS. The man, who died on May 1, developed pneumonia after undergoing surgery in the orthopedic unit of North York General Hospital. His case was linked to the original spread of SARS from Scarborough Grace Hospital, the epicenter of Toronto's outbreak in March.

The 96-year-old man, who did not show classical symptoms of the disease, became a silent SARS case, transmitting the disease to unsuspecting health care workers and other patients. As the disease lurked stealthily, protection guidelines were lifted for health care workers who were not working in SARS units and thus "became sitting ducks for this infection," an official reported. The 96-year-old man, however, was not a patient in a SARS unit, health officials said. No one suspected him until other patients began showing symptoms of atypical pneumonia that were not responding to antibiotics.

Officials thought they had contained the spread of the disease two weeks ago. Toronto was declared safe after the World Health Organization removed the city from its list of SARS-affected areas on May 14. Officials then focused on repairing the devastated economy and trying to lure tourists back to the city.

But hospital workers said they never believed SARS had been eradicated.

"The latest outbreak is a resurgence of the first outbreak," said Richard Schabas, chief of staff at York Central Hospital. "The first outbreak was not entirely extinguished. It was like an ember smoldering in a fire. You think you put it out and the next thing the wind comes up and you have a new fire."

Schabas said precautions for health care workers were lifted too soon. "But the real issue isn't that," he said. "If you looked at the data as it was known at the time, removing the precautions was a good thing to do. The problem was the data wasn't good enough. We didn't do intensive surveillance and case findings. We should have been looking very hard at every hospital in Toronto to see if we could identify not just SARS cases, but cases of pneumonia that looked like SARS. We should have zeroed in on them."

Nurses said this week that they reported to their supervisors recent cases of patients with unusual types of pneumonia. They complained that the disease had time to spread because investigations were not conducted until two days later.

A liquor store in Aurora, Ontario, was closed until further notice, authorities said today. They were trying to find out how a cashier at the liquor store contracted SARS when he visited a hospital. The patient he was seeing did not have SARS. "He had reason to visit the hospital May 23. He had a temperature before that," said Colin D'Cunha, Ontario's commissioner of public health. "He may have come in close personal contact with an infectious case on that floor." Health officials say there was minimum risk to customers in the store, but they have asked other employees to isolate themselves.

Officials also said they were investigating several reports of people in the United States who may be suffering from SARS-like symptoms after visiting Toronto, but they said they think it is unlikely the people have SARS. "There are a couple of situations in Detroit and one in Pennsylvania and one in Arkansas," D'Cunha said.

Health officials in West Virginia said they were investigating the case of a 68-year-old man who may have contracted SARS during a recent visit to Toronto. The man, who was in critical condition in Wheeling Hospital, has been classified as a probable SARS case because his symptoms fit the disease and he was in Toronto last week and a month earlier.

A WHO report from Geneva said Taiwan is close to bringing SARS under control. WHO cited improvements in infection control in hospitals, which have been the main source of outbreaks.

Taiwan posted 50 new probable SARS cases today. Taiwan has reported 660 SARS cases and 81 deaths, with 177 cases coming since a second wave of infections peaked seven days ago.

[On Friday, Taiwan reported seven new probable SARS cases, the Reuters news agency reported.]

In the United States, the Centers for Disease Control and Prevention reported that a U.S. doctor flown to Atlanta from Taiwan last Friday may not have the disease. The doctor, Chesley Richards, was thought to have contracted SARS in Taiwan, but authorities said preliminary tests indicate he does not have the coronavirus that causes SARS.

Richards remained in isolation and further tests will be conducted, health officials said.

Staff writer Rob Stein in Washington and special correspondent Tim Culpan in Taipei, Taiwan, contributed to this report.