New Orleans — Speculation mounted Wednesday about possible causes of the weekend collapse of the half-built Hard Rock Café in New Orleans, reports the CBS affiliate here, WWL-TV. As authorities shifted from rescue operations to tearing down the structure's dangling wreckage, theories surfaced about what compromised the concrete slabs that rose in tiers from the ground until they pancaked Saturday morning, killing at least two workers.

Images of the hotel under construction before and after the collapse fueled speculation about possible contractor shortcuts and engineering mistakes, and some workers voiced criticism.

Several people who worked on the $85 million project said they'd expressed concern over the structure as they built it, and possible video evidence lending weight to those concerns appeared late Tuesday on social media.

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One video, shot by a local contractor, purports to show the concrete slab above an upper level of the Hard Rock sagging to the point of bending temporary posts, called shoring jacks, that supported it.

Randy Gaspard, a local concrete contractor who posted the video, said it was shot by a worker on Thursday, two days before the collapse. Gaspard wouldn't identify the worker or say who employed him.

"Look, Papo, 'the best engineering!' Look at these large stretches (between supports) and s--t beams! (unintelligible) They're already to the point of breaking," the worker says in Spanish in the video.

"Look at how it's bent already! They couldn't remove it because it's too bent and it has too much pressure. The huge spaces without beams – look! What a very s--t structure these architects and engineers are building! ... This is seriously bad, Papa!"

Gaspard said he's been told workers had been removing the temporary posts and "when they got to less and less of them, got more and more load on 'em, they tried to tell the contractor to stop but were told to keep going.

"What it shows is that the concrete deck has so much deflection that they can't remove the shore posts," Gaspard said. "They have so much load on them, it's bending them."

New Orleans Fire Department Superintendent Tim McConnell told WWL Wednesday that officials are aware of the video and that it's become a piece of evidence in the investigation.

Federal investigators with the Occupational Safety and Health Administration began their work Saturday, within hours of the accident. The agency's inspectors were given New Orleans police body cameras to document the hotel ruins.

McConnell said any decision on whether to pursue criminal charges over the building's collapse, or the resulting deaths and injuries, would depend on what OSHA finds.

CBS News watched the video with Norma Jean Mattei, the past president of the American Society of Civil Engineers.

"It makes we wonder why was the brace damaged, so was the concrete placed not of adequate strength? Was the concrete just not strong enough?" Mattei said.

One foreman for a subcontractor on the project, who said he was off Saturday, described another flaw. He claimed contractors failed to properly install the metal pan that concrete is poured into for the building's floors.

That corrugated metal decking was positioned differently along the edge of the building above one street than elsewhere on the building. Metal sheets were meant to overlap, but instead were installed perpendicularly, the worker said -- something he suspected caused a "shearing point" and may have contributed to the collapse.

A former ironworker who requested anonymity under the advice of his attorney said he agreed with that theory and blamed a series of changes in the design that were made during construction.

"It wasn't the ironworkers. We did everything to engineered specs. I wondered if the engineers were really OK with it," said the former worker. "The whole crew joked about how bad it was. We all joked we'd never stay in that hotel."

The firms involved in the project declined to speculate about the cause of the collapse.

Brian Trascher, a spokesman for the contractor on the hotel project, Metairie, Louisiana-based Citadel Builders, questioned the origin of the video and cautioned against drawing any conclusions from it.

"I think there's a lot of Monday-morning quarterbacking going on, a lot of people trying to flex their engineering muscles because this happened," he said.

Trascher said top experts have been brought in to assess the collapse, including some who worked on the 9/11 World Trade Center collapse and the 1995 Oklahoma City bombing.

"These people don't live on Facebook for a living, they do this kind of a thing for a living, and I think we're going to follow their advice," Trascher said.

The principal engineer on the project, Heaslip Engineering, issued a statement Wednesday that described the firm as "devastated" by the collapse. James Heaslip, the firm's founder, didn't speculate in the statement over what caused the collapse.

"As a member of the design team, we are keenly interested in finding out why this incident occurred," the statement from founder James Heaslip read.

Yet another theory came from a Texas engineer who inspects bridges for that state.

Linwood "Woody" Howell, of Austin, pointed to a "scissor lift" from drone footage of the hotel wreckage. The video and other photos show the heavy lift propped up on a pile of rubble.

That Genie 1930 model weighs nearly 3,000 pounds, and Howell suspected that, like a stiletto heel, its weight could have pierced a concrete floor that was supported by only "point shores" like those shown in the worker's video, and not by beams that would have distributed the weight more evenly.

Howell said his rough calculations based on project drawings and specs from the maker of the steel decking indicated "the floor system design was pushing the structural limits of product capability, leaving little tolerance for error."

"If the scissor lift failed one section of the top floor, then the falling weight of the floor and scissor lift would collapse floors below and eventually buckle columns and pull down areas adjacent to the failed section," Howell said. "This scenario is consistent with the video of collapse and other currently available data."

Another construction expert suggested all the problems described to WWL could have been factors in the collapse.