21.   Police suspected that the explosion occured when workers mishandled a nearby urban gas main, causing the gas to leak and catch fire from a welding torch.

22.   The cause of the explosion, which occurred late Sunday, was not immediately known but investigators said they suspected a spark from a welding torch ignited the blast.

23.   The fire was ignited by sparks of the welding torches, they said.

24.   The fire was ignited by sparks from welding torches.

25.   The leaked gas was ignited by a welding torch in the subway, they said.