Yesterday afternoon, hundreds of people were evacuated from two subway trains after a fire broke out in the subway tunnels under the Lower East Side. FDNY officials told us that approximately 500 people were evacuated starting around 2:45 p.m. from J and M trains at the Delancey Street/Essex Street stop. It was unclear yesterday exactly what started the fire—but now, sources tell NBC News that the blaze could have been sparked when a cable fell across the third rail and a track rail as someone tried to steal an underground copper wire.

Deputy Assistant Chief Jim Daly told us yesterday that the fire started in a "wall mounted electrical vault box" that supplies power to the third rails. 4th Battalion Chief Keith Anderson echoed that, saying it was an electrical fire: "A lot of smoke was coming out of the grates of the subway, we believe it's likely an electric issue." That JMZ station was vented out for smoke, and eventually returned to regular service last night.

Another source told NBC that investigators think the suspected copper wire theft may be part of a larger operation—tools used for cutting wire were found in a closet-sized enclosure in the tunnel. Their source believes the theft has been going on for months, and in all, as many as two thousand feet of cable appear to be missing from the station.

A few years ago, there were a spate of wire copper thefts—and police believed at the time that the Greenpoint Warehouse Terminal fire in 2006 was started by a homeless copper thief.