NEW YORK - New York City blamed New Jersey's wetlands on Tuesday for causing a strong stink that briefly plagued the area, but the mystery lingered as an environmental scientist said the marshes should be exonerated.

A powerful smell akin to natural gas wafted over the region on Monday morning, sending at least 19 people to the hospital in both states and forcing the evacuation of New York schools and offices. Commuter rail service was briefly halted as a precaution.

A spokesman for New York City's Department of Environmental Protection pointed to New Jersey's marshes, saying New Jerseyans reported the smell first and that calls in New York later came from the south of Manhattan and then moved north.

'So it was coming up the Hudson River,' spokesman Charles Sturcken said. 'You can map it in time.'

However, other officials on both sides of the Hudson River said the cause might never be known because it dissipated after a few hours.

'Belching bog blamed for citywide gas stink' declared a New York Post headline, explaining that mercaptan, the rotten-egg-like odor smelled by New Yorkers and New Jerseyans on Monday, can be released by decaying vegetation.

Mercaptan is also the chemical added to odorless natural gas so that it can be detected in a leak.

'It was never anything that we thought was a security risk or a danger to anybody,' the mayor said.

Joan Ehrenfeld, a biology professor at New Jersey's Rutgers University, said New Jersey's wetlands commonly produce sulfur-containing gas as part of the natural process of decay. But that gas is almost immediately converted to minerals.

'I think it is highly unlikely that it was the production of hydrogen sulfide in the salt marshes,' Ehrenfeld said.

'They're not even bogs,' sniffed Ehrenfeld, explaining that the term bog refers to wetlands whose only source of water is rainfall and whose main vegetation is peat moss.

In contrast, New Jersey's 8,000-acre (32-sq-km) Meadowlands are wetlands, distinguished by grass and their mix of fresh and salt water.

(Additional reporting by Richard Cowan in Washington)