INDIANAPOLIS - If natural gas caused the deadly home explosion on Indianapolis' south side, it had to be triggered by more than just a single spark, a Purdue University expert said.

Investigators believe natural gas caused the Nov. 10 blast that killed two people and leveled homes in the Richmond Hill subdivision, and the case is being investigated as an intentional act.

Special Section: Indy Explosion

Sally Bane, an associate professor in Purdue's School of Aeronautics and Astronautics, is not involved in the investigation, but does research on gas combustion.

She said natural gas is very hard to detonate, so an explosion as powerful as the one that rocked the city's south side would have to be sparked by more than just a furnace pilot light.

"While the explosion is bad enough, it is the detonation that's really going to cause the widespread damage," Bane said. "Natural gas is so insensitive to detonation, it's really hard to get that to happen."

She said the initial explosion caused a supersonic wave, which is why so many homes in the neighborhood were destroyed or damaged.

"The wave, combustion or explosive wave, is traveling super sonically, so faster than the speed of sound, at several thousand meters a second," Bane said. "That's the kind you see in the movies. When you see the hero from the explosion, and it catches up to them and they get flung off the ground."

The supersonic wave is also why the south side explosion could be felt miles away.

"You feel the explosion three miles away, if it has affects three miles away, it's deforming garage doors three miles away, then that was some kind of detonation," Bane said.

City officials said Wednesday that 29 homes will have to be demolished due to severe damage. Four other homes will also likely be torn down after the criminal investigation is complete.