(CNN) — As rescuers search through the debris of a deadly South Florida condo collapse for a sixth day Tuesday, families and friends of those still unaccounted for are desperately seeking information about their loved ones and investigators are trying to figure out the cause.

At least 11 people have died in the partial collapse of Champlain Towers South in the beachfront community of Surfside, and 150 remain unaccounted for following the tragedy. More than 136 people have been accounted for, according to Miami-Dade Mayor Daniella Levine Cava. The deceased range in age from 26 to 82.

“It’s hard to stay composed,” Shelly Angle, whose friend Elaine Sabino lived on the building’s top floor and has not been found, told CNN on Tuesday. Angle said she is holding onto “the glimmer of hope that just maybe somebody survived.”

As the search and rescue effort continues, the safety of emergency workers at the site is raising concerns. Surfside Mayor Charles Burkett told CNN’s John Berman on Tuesday that debris had fallen from portions of the building that remain standing.

“Overnight they did have some stuff falling down from the building that’s still standing and that’s going to have to be addressed,” he said.

“They will address that, they will have to pick those pieces off, tie them to a crane, cut the steel and remove them so it’s safe for the workers to really get down in there,” the mayor said.

A six-person team of federal officials from the National Institute of Standards and Technology — consisting of scientists, structural engineers and a geotechnical engineer — is conducting a preliminary investigation of the collapsed building’s materials, history and applicable building codes at the time the condo was built, the ground surrounding the building and numerous other factors, according to an agency official.

As investigators converge on the scene, questions are being asked about the building’s structural integrity.

An April 2021 letter from the condo’s board president to residents said some damage observed in a 2018 engineer’s report, including in the garage, “has gotten significantly worse.”

“The concrete deterioration is accelerating. The roof situation got much worse, so extensive roof repairs had to be incorporated,” the letter from board President Jean Wodnicki reads.

Wodnicki further described issues facing the building: “When you can visually see the concrete spalling (cracking), that means that the rebar holding it together is rusting and deteriorating beneath the surface.”

CNN has reached out to representatives of the condominium association for further comment.

The 2018 report from an engineering firm documented severe structural damage to the concrete slab below the pool deck and “cracking and spalling” located in the parking garage. Spalling is a term used to describe areas of concrete that have cracked or crumbled.

The 2021 letter sought to explain a $15 million assessment that condo owners were to pay for repairs to the building, with payments set to begin July 1. The association approved the assessment in April to complete repairs required under the county’s 40-year recertification process, according to documents obtained by CNN.

Building residents also had raised concerns over tremors during construction of a nearby building in 2019, according to documents obtained by CNN and interviews with the family members of people missing in the collapse.

The 2018 report didn’t say whether the structure was at risk of collapse. Donna DiMaggio Berger, an attorney for the condominium board, told CNN’s Chris Cuomo on Monday that she didn’t think the building was in a state of disrepair prior to its collapse.

“We have months and years to dig into what happened, and we’re going to. The board is already in the process of hiring an engineer to also try to figure out what happened, and they will be evaluating who’s responsible.”

The investigation will take months

While there are no clear answers on the cause, early signs point to some failure in the lower reaches of the 13-story building, perhaps in its foundation, columns or underground parking garage, according to experts.

Forensic engineers will need to examine the ground-floor columns in their investigation, Sinisa Kolar, a Miami-based engineering executive, told CNN.

He said that samples of concrete will need to be tested to examine the condition and then those samples will need to be cross-referenced with structural drawings.

“The key element to this investigation, in my opinion, lies in that rubble, in those columns and condition of the structural elements,” Kolar said.

The investigation could take months or longer, according to Allyn Kilsheimer, the structural engineer hired by the town of Surfside. He has started to examine the building and will use a meticulous, computer-assisted process of elimination to attempt to identify the cause or causes, he said.

“Unless it’s a plane or a bomb that you know triggered this whole thing, sometimes you can’t get it down to one cause,” he said. “You don’t know what you’re going to end up with until you finish the whole study.”

A pool contractor who was servicing the condo pool 36 hours before its collapse told the Miami Herald he had seen standing water and cracks in concrete underneath the pool deck.

When he first entered the building, the pool area and upper lobby looked fine, but when he walked into the basement garage under the pool deck, he claims he saw lots of standing water and claims to have found cracks in the concrete in the pool equipment room, according to Miami Herald reporter Sarah Blaskey.

“He took a picture of that concrete to send to his boss, because he was there to do a little bit of cosmetic stuff to the pool, but he thought, ‘Wow, this is going to be a bigger job,’ took a picture, sent it to his boss,” Blaskey told CNN.

She noted that the photos were reportedly taken in the equipment room on south side of the basement structure, rather than the north side, where the structure caved.

Families and friends are waiting for answers

Meanwhile, at least 11 victims have been identified. They are Stacie Fang, 54, Antonio Lozano, 82, Gladys Lozano, 80, Manuel LaFont, 54, Leon Oliwkowicz, 80, Luis Bermudez, 26, Anna Ortiz, 46, Christina Beatriz Elvira, 74, Marcus Joseph Guara, 52, Frank Kleiman, 55 and Michael David Altman, 50.

Family members of those still unaccounted for are waiting for any answers they can get as the search continues.

Rachel Spiegel, whose mother Judy Spiegel is unaccounted for, said it’s been difficult to relay to Rachel’s 4-year-old daughter what has been happening. Rachel told her daughter that there was an accident in the building, and that “grandma is missing.”

“And she told me, ‘I can go get her. … I know where she hides. I’m really good at finding her,'” Rachel Spiegel told CNN Tuesday morning. “When she’s telling me that, I’m … crying even more, but at the same time trying to keep my composure because I don’t want my 4-year-old to worry too much.”

“I hope my mom is alive … but we also are very aware of the reality of the situation and the time — I mean, it’s Day 6. And the fact that they aren’t finding people is of tremendous concern,” Rachel Spiegel said.

Ronit Felszer and Carlos Naibryf told CNN’s Alisyn Camerota on Monday that they are being realistic about the possible outcome.

Carlos Naibryf said his son “was a 21-year-old young adult. Bright. Everywhere he went, he made an impact … now he’s at U. Chicago, physics major.”

When Camerota asked the parents if they’re still believing in a miracle, Felszer shook her head.

“We want to believe in a miracle because we still don’t have the physical presence, in part or in whole, of our son,” she said.

“A miracle can come, yes, but we have to be very realistic,” the father told CNN.

Rescue conditions remain dangerous

While families grieve and hope and officials investigate, Mayor Burkett said the primary focus is to get victims out of the debris.

“The number one priority here is pulling these people out of the rubble and we’re going to focus only on that. The second priority is supporting the families,” Burkett said Monday night.

Miami Dade Fire Chief Alan Cominsky said the search efforts are “a slow process, but we’re definitely making progress.” Rescuers are working 12-hour shifts and efforts have continued non-stop since the collapse early Thursday morning.

“These men and women aren’t pausing,” Florida CFO and state Fire Marshal Jimmy Patronis said. “They continue to have a type of momentum and motivation that is really, really inspiring

Victims have been found in different areas as the teams search through various search grids, rather than one isolated area, according to Cominsky.

While rescuers were impeded by weather conditions and smoke conditions earlier in the week, conditions have improved but remain dangerous in the search area.

“There’s definitely a concern with the rain, and now the debris, and possibly sliding — so I mean it’s an extremely dangerous situation,” he said. “It’s a very, very difficult, very, very challenging situation, and you know we’re doing the best that we can, what we’re trained to do and will definitely continue moving forward.

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