ST. PETERSBURG, Fla. — From sinkholes to concrete-weakening saltwater intrusion, theories abound on what caused a Florida beachside condominium tower to collapse with no warning.

The Champlain Towers South building in Surfside that partially went down was in the midst of its 40-year recertification process, which requires detailed structural and electrical inspections.

Surfside Mayor Charles Burkett said in an interview Friday said he wasn't sure the inspection had been completed but may contain vital clues to the deadly disaster.

"It should have been a very straightforward thing," Burkett said. "Buildings in America do not just fall down like this. There is a reason. We need to find out what that reason is."

The 12-story tower's collapse has left at least four people dead, 159 missing as of Friday and numerous questions about how this could have happened — and whether other similar buildings are in danger.

Details of the Champlain Towers recertification inspection will be made public once they are completed, Surfside Town Clerk Sandra McCready said in an email. The engineer involved in the building's recertification process, Frank Morabito, did not respond to telephone messages and emails requesting comment Friday.

Miami-Dade Mayor Daniella Levine Cava said at a news conference Friday that she has seen no evidence of a sinkhole — much more common in other parts of Florida — or of something criminal, such as a bomb.

"I can tell you that at this time, they haven't found any evidence of foul play," she said.

Beyond that, much focus is on ocean water, which is rising in South Florida and elsewhere because of climate change. Last year, Republican Gov. Ron DeSantis signed into law a measure that would require developers to complete sea-level rise studies before beginning publicly funded projects.

Like everyone else, the governor wants answers about the cause of the collapse as soon as possible.

"We need a definitive answer for how this might have happened," DeSantis said at a news conference. "It really is a unique type of tragedy to have, in the middle of the night, half a building just collapse like that."

Meanwhile, the land on which Champlain Towers sits has been gradually sinking, according to a study published last year by an environmental professor at Florida International University.

But the professor, Shimon Wdowinski, cautioned against blaming the collapse on the caving ground. The study used satellite data collected between 1993 to 1999 to study the sinking of land in Norfolk, Virginia, and Miami Beach.

In a video interview released by the university, Wdowinski said his study found numerous examples of sinking earth, some leading to cracks in buildings — which he called "pretty common" in Florida. 

"In most cases, these buildings just move," he said, "there's no catastrophic collapse like in the case in Surfside, which was very unfortunate."

Another theory is that the saltwater ubiquitous in the area, which is subject to flooding during so-called King Tide events, intrudes into concrete supports, corrodes the steel-reinforcing rebar inside and weakens the concrete.

Abi Aghayere, an engineering researcher at Drexel University, said determining if there was such deterioration could be one key to the collapse.

"Did a column fail by itself? This column has been carrying this load for 40 years, why would it fail now?" said Aghayere, adding that it is rare for rebar to be corroded without anyone noticing. "You will have concrete popping out, falling out."

Others have cited frequent flooding in the building's lower parking garage, including the possibility of water seeping up underneath through the porous limestone rock on which the barrier island sits that includes Surfside and Miami Beach.

Surfside officials say roof work was ongoing at the now-collapsed tower and have downplayed the possibility that work was a cause. Barry Cohen, a lawyer who escaped the crippled Champlain Towers building with his wife, said the roof work could be part of a "perfect storm" of causes that combined to bring down the structure.

"They were doing a new roof. And I think, all day long, the building was pounding and pounding and pounding. They've been doing it for over a month," Cohen said.

Another issue cited by some people is construction at a nearby building that might cause vibrations that weakened Champlain Towers. Cohen said he raised concerns previously that the work was possibly causing cracked pavers on the pool deck.

The collapse is already drawing lawsuits, including one filed hours after the collapse by attorney Brad Sohn against the condo's homeowners association seeking damages for negligence and other reasons for all of the tower's residents.

The association, the lawsuit contends, "could have prevented the collapse of Champlain Towers South through the exercise of ordinary care, safety measures and oversight."

An attorney for the association, Ken Direktor, did not respond Friday to an email requesting comment.