A noxious smell that wafted over New York City, Westchester and New Jersey dissipated Monday evening, but questions linger about its cause.

The strong odor sparked fears of a natural gas leak and caused city callers to flood 911 centers. However, Mayor Michael Bloomberg (R-NYC) reassured residents there was no danger. Bloomberg said while natural gas is odorless, the smell may have been caused by the common additive mercaptan. The additive allows a leak to be detected by smell. Con Edison confirms it found a small leak, but said it was not significant enough to cause the widespread smell. Officials say at least seven people were sent to the hospital.

Yonkers firefighters responded to several local calls about the odor as a precaution. The investigation is ongoing.