NEW YORK -- They bombarded 911 with calls, crowded the sidewalks in front of evacuated buildings and tuned to the news for word of what was happening. The question on the minds of so many New Yorkers: "What's that smell?"

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Images: Gas Odor Detected In NYC

Raw Video: Bloomberg Discusses Gas Odor

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A gas-like odor hung over much of Manhattan and parts of New Jersey on Monday morning, leaving many unanswered questions in its pungent wake as officials struggled to explain the origin of the smell. The odor seemed to be gone by early afternoon.

The stench set off concern from Washington Heights to the West Side Highway, down to Battery Park City and across the Hudson River. It disrupted mass transit and caused some brief evacuations and hospitalizations.

Mayor Michael Bloomberg said there was no indication the air was unsafe to breathe. "It may just be an unpleasant smell," he said. He said sensors did not show an unusually high concentration of natural gas.

Bloomberg said natural gas is odorless, and the chemical that is added to it gives it the recognizable odor. He said the smell may have resulted from a leak of that substance, called mercaptan.

Charles Sturcken, a spokesman for the city Department of Environmental Protection, said later that the odor could have been caused by mercaptan but added, "Nothing has been confirmed. We're left with a mystery, although we know it's not harmful."

Fire Department spokesman Tony Sclafani said 12 people were taken to hospitals with minor odor-related complaints such as irritation and difficulty breathing.

The smell caused some commuting problems. The Port Authority of New York and New Jersey suspended PATH train service for about an hour between New Jersey and 33rd Street in Manhattan.

Terrorism was not a concern, a federal official said.

"There is no indication of terrorism and there is no credible intelligence to suggest any imminent threat to the homeland or to New York at this time," said Russ Knocke, spokesman for the Department of Homeland Security.

The Fire Department began getting calls about the odor around 9 a.m., said spokesman Tim Hinchey.

A handful of city schools were briefly evacuated. Steve Satin, the principal of Norman Thomas High School on East 33rd Street, said the school was evacuated for about 35 minutes but then students returned to their classrooms.

Susan Badger, a retiree who lives in Chelsea, said she left her apartment building at 27th Street and Eighth Avenue to escape the smell.

"If it's throughout the whole city, it seems that it must be a lot of gas. It's really extreme," she said.

Bill Battipaglia, a clerk at a U.S. Postal Service facility in Chelsea, said the smell seemed to be coming from the building's ventilation system between 9:30 and 10 a.m.

"It was strong enough for me to leave," Battipaglia said, standing outside the building on Ninth Avenue.

"That smell was stinking. It smelled like, toxic," said Alfred Stewart, 47, who lives in a Chelsea apartment building near 10th Avenue. He thought it smelled like a mix of oil and kerosene.

"You stayed in it and held it enough, you probably would have got dizzy from it," he said.

The smell was reported as far downtown as Battery Park City, where the management at one apartment building shut off the outside air supply and advised tenants to close their windows.

Complaints about the odor also came from Bergen and Hudson counties in New Jersey, she said. But no air sampling was done in New Jersey because the state Department of Environmental Protection received no specific location "to hone in on," spokeswoman Elaine Makatura said.

Tim Morrin, a meteorologist at the National Weather Service, said winds were fairly light, about 5-10 mph, coming from the south/southwest.

"There are no abnormal changes in the gas flow in our gas transmission system. If there was a big leak, we would see a change in the gas flow," said Con Edison spokesman Chris Olert.

He said more than 60 Con Edison workers fanned out all across Manhattan's west side, but found no indication of a gas leak. He said the company fielded 700 calls from people concerned about the odor, from as far north as Washington Heights to as far south as Greenwich Village and as far east as Lexington Avenue.

New York City is no stranger to odd stenches wafting through its streets.

In 2005, a maple syrup aroma spread across Manhattan twice within a few weeks. Environmental officials sent teams into neighborhoods where the calls originated but found nothing dangerous and could not explain the odor.

Last August, seven people were treated for headaches and nausea after a gaseous odor was reported in Queens and Staten Island. The odor passed without any answers about its origin.