(CNN) -- It must be a bomb.
That was Debbie Wagner's first thought when she heard the boom as she lay in bed overnight Saturday in her home on Indianapolis' Southside. Downstairs, she found a ceiling fan plunged to the floor, glass shattered, and the deadbolt of her front door blown open -- not even knowing then her garage doors had been blown in.
Only later did Wagner and her husband Andy learn that, among their neighbors, they were some of the lucky ones. A huge explosion had pierced the nighttime silence and ripped through the streets before characterized by solid two-story homes, garages and lawns.
Two adults were killed, while seven others went to area hospitals, due to the explosion and related fires, said Deputy Chief Kenny Bacon of the Indianapolis Fire Department.
Aerial video showed the significant material cost as well, with only the foundations remaining of some homes, others left charred, and dozens more homes like the Wagners' suffering damage of all different sorts.
"The house felt like it was shaking, your chest was shaking," Wagner told CNN affiliate WISH. "It's like nothing that you've ever experienced."
Fire department spokeswoman Bonnie Hensley said early Sunday that two houses sitting next to one another blew up. But hours later, authorities still hadn't determined, publicly at least, why they did. Citizens Energy spokesman Dan Considine said Sunday there were no reports of gas smells prior to the blast.
While the personal and physical toll was enormous, some officials are counting their blessings and crediting the prompt, extensive response from public safety authorities -- plus the good fortune that some locals were away for the night -- with preventing even more bloodshed.
"It was a massive explosion," said Troy Riggs, the city's director of public safety. "... But it could have been much worse."
Soon after it happened, first responders swarmed on the area -- some of them in shorts and tennis shoes, having apparently rushed to the scene right away, said Riggs.
At one point, 60 to 70 firefighters were on site to control blazes and prevent more, according to Bacon. Their other primary duty was to round up residents, as crews "did at least a primary, secondary and tertiary search of every home" to clear people out, he said.
About 200 neighbors eventually made their way to nearby Mary Bryan Elementary School, where doctors and paramedics checked each one, said Bacon.
Wagner described the predominant feeling among her neighbors as sorrow.
"People are just ... in shock, because it is such a surreal thing," she said.
After the sun rose Sunday morning, the devastation became more stark and painful. Authorities went door to door at 126 properties in the neighborhood, dividing them into a few simple categories: "OK" for those with cosmetic and survivable damage, other designations for those with an uncertain future, or those that were a lost cause.
Eighty homes were somehow affected, said Adam Collins, the deputy code enforcement director for the city, causing an estimated $3.6 million in damage. Most of these had minor damage, in the form of broken glass, torn-off siding or garage door damage, that didn't make their homes uninhabitable.
Another 31 suffered what Collins called "major damage" -- meaning, for most of them, it's still to be determined if they can be lived in again. Inspectors know for sure that five cannot, as they are "either gone or will require a demolition."
Riggs said he expects authorities will discover more damage, and residents will have to deal with more headaches and heartache in the coming weeks. Handing out his cell phone number to those affected, he promised them they wouldn't be alone.
"We understand that their lives are upside down," Riggs said. "We're going to do everything we can to help (re-)establish their lives, and go forward."
CNN's Jake Carpenter, AnneClaire Stapleton and Greg Botelho contributed to this report.