It was the odor associated with natural gas - the telltale, unpleasant sulfur scent that typically signals a gas leak. But this time, it was lingering in many areas of Manhattan and northeastern New Jersey, coursing through buildings and leading to fears that it could ignite or that a dangerous chemical had been deliberately released.

Schools and office buildings were evacuated. A subway station was shut, and commuter trains were rerouted. Government security officials were put on alert. Fire trucks raced through the streets, while Coast Guard vessels patrolled New York Harbor, communicating with tugboats and container ships. Twelve people with complaints of minor illnesses or injuries were taken to hospitals.

The source of the odor? As of last night, city officials still did not know. But it lingered for an hour after first being reported around 9 a.m., leaving New York with another mystery on its hands and more than a few conspiracy theories to sort through.

With anxieties about gas leaks rattling the nerves of the city, Mayor Michael R. Bloomberg held a press conference to assure residents that the city's air-quality detectors had found no cause for alarm. He hypothesized that the odor could have been caused by the release of mercaptan, a compound that smells like rotting eggs and is added to natural gas so people can detect and report leaks.

Throughout the day, possible culprits - among them a minor gas leak in Greenwich Village and natural-gas pipelines in northeastern New Jersey - were considered and ruled out.

The olfactory mystery in the New York region was matched by strange activity elsewhere. In Austin, Tex., police cordoned off 10 blocks of the downtown business district early yesterday after more than 60 birds were found dead overnight along Congress Avenue, which leads to the State Capitol. Air testing there failed to find a cause, but preliminary results determined that people were not at risk.

In New York, the piercing odor was the talk of Manhattan, and it called to mind another mystery: the maple syrup odor that people reported smelling on separate days in late 2005 and whose source has never been established. In yesterday's case, several people said they were overcome by the odor.

"I feel faint," said Ivolett Bredwood, a legal assistant who noticed the odor once she stepped off a New Jersey Transit train at Pennsylvania Station around 8:45 a.m. The smell trailed her as she walked to her office, at 99 Park Avenue, which was briefly evacuated. "It's an awful, nasty smell."

The widespread uncertainty and potential for danger led the authorities to take numerous precautions as thousands of reports of the odor flooded into 911 and utility hot lines.

The Metropolitan Transportation Authority briefly closed the subway station at 23rd Street and Avenue of the Americas, as well as a control tower at West Fourth Street. Service was temporarily halted on PATH lines terminating at 33rd Street.

The major gas utilities - Consolidated Edison in New York and Public Service Electric and Gas in New Jersey - checked their transmission lines and reported no leaks, changes in pressure or other abnormalities.

The city's Department of Environmental Protection dispatched a mobile laboratory to the West Side with meters to test for ammonia, chloride, cyanide, methane, carbon monoxide, hydrogen sulfide and volatile organic compounds. "That's the hardest part, finding the source," said Christopher Haas, a department specialist in hazardous materials. "Air is very dynamic."

Officials were reluctant to discuss terrorism precautions in great detail, but they said that the city regularly monitors the air with machines that can detect the presence of chemical, biological or radiological substances.

At the Port Authority Bus Terminal, some alarmed passengers thought that their buses had problems. And at the Equitable Center, on Seventh Avenue between 51st and 52nd Streets, air vents were closed to keep the odor out. Two schools were evacuated. Norman Thomas High School in Midtown was emptied for about 50 minutes beginning at 9:30 a.m., while students at Public School 11 in Chelsea were taken to Public School 33 nearby.

Jeremy Fleishman, a worker at a computer repair shop in Chelsea, said it smelled as if "somebody left the Bunsen burner on" in chemistry class. By 10:30 a.m., he said, "it mostly dissipated - or maybe we just got used to it."

At 980 Avenue of the Americas, a building that was briefly evacuated, a guard, Ralph Supino of Secaucus, N.J., said he called Con Edison but reached only recorded messages. "They were overwhelmed," he said.

For some, it seemed logical that the odor was tied to some sort of terrorist plot. At 1250 Broadway, which was also briefly closed, a guard, Miguel Contreras of Irvington, N.J., said that thought raced through his mind when he noticed the smell upon arriving at the bus terminal on his way to work.

"You pray to God that everything is fine and it's just a leak somewhere," he said.

Adding to the alarm was the strength and duration of the odor, which may have been aggravated by a weather phenomenon known as a temperature inversion. Inversions, which often occur when a warm front moves over a cooler, denser air mass, cause the temperature closer to the ground to be cooler and the air higher up to be warmer - a reversal of the usual pattern. Inversions can trap pollutants and odors, preventing them from being dispersed upward.

David Wally, a meteorologist at the National Weather Service's forecast office in Upton, N.Y., said a warm front approached the city between 7 and 8 a.m., making it "very possible" that an inversion trapped the pollutants and gaseous odor closer to the ground. The inversion eroded later in the morning, he said.

The city recorded 4,500 more 911 calls than usual between 9 and 11 a.m., with most of the increase in Manhattan. The Fire Department responded to 450 calls, 41 of them for emergency medical assistance.

Dr. Kristin E. Harkin, an emergency-medicine physician at NewYork-Presbyterian Hospital/Weill Cornell Medical Center, said that strong odors can worsen the symptoms of people with chronic respiratory ailments like asthma and emphysema.

Some suspicion fell on New Jersey, given the path of the prevailing winds and the prevalence of chemical and petroleum facilities in the state. Calls about the smell were received in West New York,Weehawken and other places.

In Hoboken, the downtown police headquarters and several office buildings were briefly evacuated, according to Mayor David Roberts, who said he took an anxious call about the smell from his wife.

Jack Burns, coordinator of the Hudson County Office of Emergency Management, in Secaucus, said that officials had ruled out the possibility of a mercaptan spill there. He added, "If it's in New York and people can smell it in western Hudson County, that's a lot of it, whatever it is."

Michael Williams, an accountant in Jersey City, said he delayed taking a smoking break for more than an hour because the odor was so intense. "I didn't want to spark an explosion or anything," he said.