Source?
It's the name of what's added to natural gas... to make it discernible.
"The mayor noted that natural gas is odorless and colorless. However, a chemical called mercaptan is added to the substance to make sure it can be detected. He said just a small amount can create an odor. "
"'It is still early but 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.'
He said the agency is closely monitoring the situation and talking with the local authorities, who are the lead in the matter."