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 many New Yorkers on Monday morning was: "What's that smell?"

Workers who left their offices after smelling a mysterious gas-like odor are seen standing outside of the Time & Life Building in New York, Monday, Jan. 8, 2007. [AP]

A natural gas-like odor hung over much of Manhattan and parts of New Jersey, confounding authorities. The smell seemed to be gone by early afternoon.

Mayor Michael Bloomberg said there was no indication the air was unsafe. "It may just be an unpleasant smell," he said. He said sensors did not show an unusually high concentration of natural gas, and the city's major utility company reported it found no gas leaks.

The mayor said the smell may have been caused by a leak of a substance called mercaptan that is added to natural gas for safety reasons to give it a recognizable odor. By itself, natural gas is odorless.

Some commuter trains running between New Jersey and Manhattan were suspended for about an hour as a precaution. A few city schools were briefly evacuated. Some apartment dwellers were advised to close their windows.

Homeland Security spokesman Russ Knocke said there was nothing to suggest terrorism.

"That smell was stinking. It smelled like, toxic," said Alfred Stewart, 47, who lives in an apartment in Manhattan's Chelsea section. He said 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."

Twelve people were taken to hospitals with minor complaints such as irritation and difficulty breathing, fire officials said.

The Fire Department began getting calls around 9 a.m. Gas provider Con Edison said it fielded 700 calls from people worried about the smell, from as far north as Washington Heights to as far south as Greenwich Village and as far east as Lexington Avenue.

Con Edison spokesman Chris Olert said more than 60 utility workers fanned out across Manhattan's West Side but found no indication of a gas leak.

Norman Thomas High School on East 33rd Street was evacuated for about a half-hour.

Susan Badger, a retiree who lives in Chelsea, said she left her apartment building 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.

Complaints about the odor also came from New Jersey, across the Hudson River from Manhattan. But no air sampling was done there because officials had no specific location to investigate, said Elaine Makatura, a spokeswoman for the state Department of Environmental Protection.

New York City is no stranger to odd smells.

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

Last August, seven people were treated for headaches and nausea after a gaseous odor was reported in Queens and Staten Island. Its source remains a mystery.