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The fact that rescuers heard banging sounds from beneath the collapsed Champlain Towers South apartment building in Surfside, Fla., just outside Miami, is a good sign that they may yet find additional survivors, according to a former FDNY member who responded to both terror attacks on the World Trade Center in New York City.

"The key to survival is whether or not there's gonna be what we call voids," said Vincent Pickford, a retired FDNY lieutenant and expert on confined space rescue operations. "What they're looking for are pieces of the building that kind of land at right angles to each other so that it creates spaces where people could survive."

To find them, rescuers deploy sonar equipment in a perimeter around the fallen structure, he said. The devices communicate with one another to help locate possible survivors and send audio to a human operating the equipment.

(Reliable News Media)

And if there are no other explanations for the banging sounds, then they are good signs that survivors are trapped in those voids and trying to attract the attention of rescuers, who use sonar equipment to try and locate victims within the rubble, he said.

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"Typically, when victims make noise, it tends to be like a rhythmic sound," he said. "Like banging a pipe every 10 seconds. It's not just some noise that would be repeated for some other reason. You would be listening for something that would be human-made, and you would know the difference."

The search and rescue effort had been underway for nearly 20 hours as Thursday evening. At least one person had been confirmed dead and 99 were unaccounted for – with rescuers saving more than three dozen people, including at least three who were trapped in the debris.

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Pickford said he expected it to continue for up to 48 hours.

This aerial photo shows part of the 12-story oceanfront Champlain Towers South Condo that collapsed early Thursday, June 24, 2021 in Surfside, Fla.  

This aerial photo shows part of the 12-story oceanfront Champlain Towers South Condo that collapsed early Thursday, June 24, 2021 in Surfside, Fla.   (Amy Beth Bennett /South Florida Sun-Sentinel via AP)

"If you think you have people, you're gonna keep this a rescue operation, and you're going to see selected debris remove, nothing big, but if we get this slab out of the way, it might open up a tunnel that we could then maybe get to some people," he said.

Video of the collapse shows two wings of the building give way – seemingly from the bottom up.

Pickford said it could have been from a sinkhole or something unstable underground, but investigators have not yet determined what caused the high rise to fall.

"Buildings like this do not fall in America," Surfside Mayor Charles Burkett told Fox News’ Tucker Carlson Thursday night. "This is a third-world phenomenon, and it’s shocking."

Rescue workers look through the rubble where a wing of a 12-story beachfront condo building collapsed, Thursday, June 24, 2021, in the Surfside area of Miami. (AP Photo/Lynne Sladky)

Rescue workers look through the rubble where a wing of a 12-story beachfront condo building collapsed, Thursday, June 24, 2021, in the Surfside area of Miami. (AP Photo/Lynne Sladky)

He said the "disturbing" collapse was reminiscent of the fall of the Twin Towers in New York City following the Sept. 11, 2001 terror attacks.

Pickford also noted the similarity.

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"To me, it seemed like it fell because it was being undermined," he said. "It all came down in one section, sort of like the Trade Center did, and that was because the floors underneath each section, they got weak."

"Florida's prone to sinkholes," he added. "It seemed like it had that same kind of feel to it, that something underneath was no good."