Investigators believe a gas appliance may have been the cause of a powerful explosion that engulfed a middle class neighborhood in flames. NBC's Ron Allen reports.

With no hint of a problem in advance, in particular no tell-tale smell of a gas leak, authorities and residents in a southern Indianapolis neighborhood are trying to make sense of an enormous blast that obliterated two homes and made dozens more uninhabitable.

Fire officials expressed amazement that only two people died in the late Saturday explosion so powerful that the devastation spread for blocks from its epicenter. Hundreds of residents were forced to evacuate their Richmond Hill homes, some never to return. Windows and doors were blown in. The blast rocked several houses entirely from their foundations and was so loud it awoke people three miles away. A fire burned for hours, engulfing dozens of homes.

“We have done initial testing throughout the neighborhood and have not found any gas leaks," Dan Considine, Citizens Energy spokesman, told IndyStar.com Monday.

"We are still doing additional testing of the gas main and the lines to the homes on Fieldfare Way,” he said. “We have not at this point found any problems with any external gas lines.”

The Bureau of Alcohol, Tobacco, Firearms and Explosives was also called in to help with the investigation.

Early Monday, Indianapolis public safety director Troy Riggs said forensic investigators were talking with utility companies and others as they tried to determine the cause.

"We need to make sure that we get some of the forensics back and that we follow where the evidence takes us," Riggs told WISH-TV.

U.S. Rep. Andre Carson, who represents the area, has said he had been told a bomb or meth lab explosion had been ruled out.

Deputy Fire Chief Kenny Bacon said investigators hadn't ruled out any possible causes.

Citizens Energy had received no calls from people in the area smelling the rotten eggs of a chemical added to natural gas, which is odorless, utility spokesman Dan Considine said.

"Most of the time when there's a gas leak, people smell it," he said. "But not always."

Carson said the National Transportation Safety Board and the federal Department of Transportation, which have oversight over pipelines, were sending investigators.

Riggs said police officers and investigators would continue to search and secure the neighborhood on Monday.

"It could take some time. We've asked people to be patient," Riggs told WRTV.

Dan Able, a 58-year-old state employee who lives across the street from the flattened homes, was puzzled by the blast.

"I'm wondering about all the possibilities it could be," he said.

Authorities set up relief operations at a school and church to shelter those displaced in the blast. Some moved in with friends and relatives. Others found hotel rooms.

Alex Pflanzer was sound asleep when the explosion blew out his windows and his wife started to scream.

"I didn't know what was going on," Pflanzer said. "I thought someone was breaking in the house, because the alarm was going off."

Pflanzer grabbed his gun and checked the house. Then he noticed the front door was open and saw a reddish glow flickering outside.

"I walked outside and all the houses were on fire," he said.

The Pflanzers and their two dogs found a hotel room, but they couldn't coax their panic-stricken cat out of a crawlspace.

"All the material things can be replaced, so I'm not worried about that stuff," he said. "People are a lot worse off than I am. People died, and so our thoughts and prayers go out to them first."

Officials have not released the identities of the two people killed. A candlelight vigil was held Sunday night at Southwest Elementary School in nearby Greenwood for second-grade teacher Jennifer Longworth. She and her husband, John Dion Longworth, lived in one of the homes destroyed in the blast. WTHR-TV reported that friends, family and colleagues of the teacher gathered at the school.

Deputy Code Enforcement Director Adam Collins said 80 homes were damaged including 31 that might need to be demolished. He estimated the damage at $3.6 million.

Some residents were allowed to return to their homes under police escort Sunday, but just to retrieve a few belongings. Others whose homes weren't as badly damaged were allowed to return, but officials said they would have to do without electricity overnight. And others, officials said, never will be allowed to go back inside their homes.

"There are houses that will have to be torn down," Bacon said.

He said the toll could have been much worse. "I know we're very fortunate that some of the people weren't home," he said.

Officials said at least two dozen off-duty police officers who live on the city's south side rushed to the scene to help with the rescue effort. More than 80 firefighters battled flames and searched the wreckage of homes for trapped survivors.

At the Southport Presbyterian Church shelter, tables were piled high with blankets, food, diapers, water and other supplies. An animal shelter on the south side of the city offered free boarding for pets whose owners had nowhere to take them.

Rev. Rob Hock said hundreds of congregants had shown up to help after he put out a call during Sunday morning services.

Darron Cummings / AP The powerful nighttime blast shattered windows, crumpled walls and could be felt at least three miles away.

NBC News staff contributed to this report.

More content from NBCNews.com:

Follow US news from NBCNews.com on Twitter and Facebook