More than four years after a Florida condominium collapse killed 98 people, federal investigators have yet to make a final determination of the cause — but they do have some leading theories.

The National Institute of Standards and Technology, the agency handling the probe, said this week it hopes to conclude the investigation in 2026.

“We intend for our investigation of this failure to have a lasting impact, save future lives and ensure this never happens again,” NIST investigator Judith Mitrani-Reiser said in the agency’s latest report.

Most residents were asleep in the 12-story Champlain Towers South when the beachfront condo building in Surfside, Florida, collapsed into a huge pile of rubble at 1:22 a.m. on June 24, 2021. As the investigation continued, a Miami judge approved a more than $1 billion settlement for personal injury and wrongful death claims from the disaster.

Meanwhile, a new luxury condominium is going up at the Champlain Towers site, a few miles north of Miami.

What caused the collapse?

NIST has zeroed in on what it calls three “higher-likelihood” scenarios, all related to construction flaws that date to the beginning of the 40-year-old structure.

“These conditions existed from the time construction was complete, 40 years before the partial collapse,” said Glen Bell, co-lead investigator on NIST’s National Construction Safety Team.

One possibility is the failure of a connection between a building column and the pool deck slab that never met building code standards. Another is that steel reinforcement “was not placed where it should have been,” which meant the column and pool deck were far too weak.

And a third theory is that work done later around the pool — when heavy planters, sand and pavers were added — increased the weight load on a deck “that was already functionally and structurally inadequate.”

The NIST report also notes that support columns in the building’s basement parking garage had been exposed to frequent flooding, which causes corrosion in steel reinforcements and concrete deterioration.

Are there other theories?

Investigators did not find evidence of voids in the ground under the building, known as “karst.” Using satellite data, the NIST team found there was no sinking or settling underneath Champlain Towers, which would indicate existence of karst.

In addition, investigators found the limestone upon which the condo was built was “sufficient to carry the building loads” and that testing of the concrete supporting Champlain Towers had “adequate material strength.”

One challenge for investigators was the lack of any available records from the original building construction and not many from its earlier years.

How are other condo buildings handling new rules?

After Surfside, state legislators enacted a law in 2022 requiring condo associations to have sufficient reserves to cover major repairs. Some residents were caught off guard by hefty fees imposed to cover years of deferred maintenance expenses required to bring their buildings into compliance with the law’s standards.

Gov. Ron DeSantis this week signed new legislation allowing some condo associations to fund their reserves through a loan or line of credit. It also gives residents more flexibility to pause payments into reserve funds while they prioritize needed repairs. It extends the deadline for condo associations to complete structural integrity studies and exempts some smaller buildings from those studies.

“Now it’s time to make the change,” state Sen. Ed Hooper said. “Elderly people are losing their condos because they could not afford to make the increase in their monthly HOA fees. That’s just wrong.”

What is being built on the site?

It’s a luxury condo building, dubbed the Delmore, with 37 “mansions in the sky” and a starting price of $15 million. The site was purchased at auction by Dubai-based DAMAC International for $120 million.

According to the company, there will be amenities such as a see-through swimming pool, an indoor pool, an outdoor kitchen, a fitness center and a meditation garden.

The new condo is expected to be completed by 2029.

Will there be a memorial to the victims?

A design for a memorial to the 98 victims and that honors their families was approved earlier this year by Surfside officials and a family committee. But the city’s planning and zoning board objected to its appearance and recommended that alternatives be considered.

The existing proposal envisions a tall “wall of water” and exhibition of materials from the collapsed building. Surfside officials say they want to keep the project on track but will consider additional input, especially from the family committee.

“I understand the urgency. But this memorial is going to be here long beyond anyone in this room,” said planning board chair Lindsay Lecour at an April city commission meeting.