Twelve people were taken to hospital with symptoms of shortness of breath and skin irritation after a foul smell spread over Manhattan and neighbouring north-eastern New Jersey for several hours.
The foul smell, that spread on Monday, raised fears that it might have been deliberately released. Though, despite efforts by law enforcement agencies, its source could not be located.
Emergency services received thousands of calls as the odour wafted across Manhattan. Authorities quickly moved to allay fears with Mayor Michael Bloomberg saying it is not dangerous, and the Homeland Security Department asserting terrorism is not involved.
However, no one could explain the phenomenon after Consolidated Edison, which is major supplier of electricity and natural gas in Manhattan, said its transmission lines do not show any abnormal fall in pressure, indicating that there was no gas leak.
In New Jersey, the Public Service Electric and Gas too said there was no leak in their system after running tests.
For odour to cover so much area, a major gas leak would be needed, they added.
The city Department of Environment sent mobile teams to test air quality and later declared that there was nothing dangerous.
In an apparently unrelated incident, more than 60 birds were found dead in Austin, Texas, along Congress Avenue which leads to the state capital, and air test failed to determine the cause.
In Manhattan, the odour was so strong that several high rise residential and office buildings and schools were evacuated with residents thinking that gas was leaking from their building.
Commuter trains were halted for sometime and some were diverted. The Coast Guard stepped up patrolling along New York harbours.
At a press conference, Bloomberg conceded that the source of the nerve rattling odour was not known, but put forward a hypothesis that it could be leak of mercaptan which smells like rotten eggs.
Natural gas has no odour and additives like mercaptan are added so that leaks could be detected.
The New York Times recalled another mystery which was never solved; a maple syrup odour that people reported on different days in late 2005.
Meteorologists appearing on television said the duration might have been long because of what they called temperature inversion phenomenon, which occurs when cooler, denser air is trapped by warm air moving over it.
Such a phenomenon could trap gas nearer ground, they but agreed that there has to be a leak somewhere.
