The death toll from the collapse of a 12-storey apartment tower north of Miami rose to nine on Sunday as search-and-rescue teams continued to pick through the rubble for a fourth day in the hope of finding survivors.

Charles Burkett, mayor of the town of Surfside, promised residents that authorities were wholly focused on rescue operations, but needed more luck. “We are not resource-poor,” Burkett said. “We don’t have a resource problem, we’ve had a luck problem. We just need to start to get a little more lucky right now.”

Burkett said search-and-rescue teams had made “substantial” progress overnight. Officials said a fire burning within the rubble and hampering search efforts was extinguished at around noon on Saturday.

Rescue efforts were reinforced by teams from Israel and Mexico. A 125-foot long, 20-foot wide and 40-foot deep trench was constructed at the site overnight, allowing rescuers to find more bodies and human remains.

The collapse of the Champlain Towers South building on Thursday has left 152 people unaccounted for, and fuelled anxieties about the safety of other apartment buildings, especially the nearby sister building Champlain Towers North. Burkett said on Saturday that he had sought an emergency inspection of that building. *

County officials said they would carry out sweeping safety reviews of older buildings. Daniella Levine Cava, mayor of Miami-Dade County, said there would be a “deep dive” over the next 30 days to assess buildings approaching 40 years of age or older.

That review, however, would not include buildings in cities, which have their own powers, Levine Cava said, speaking on CBS News.

The New York Times on Saturday reported that an engineering consultant found alarming evidence of “major structural damage” at the collapsed Champlain Towers South in 2018. Burkett said on Sunday that city authorities would be looking “very, very, very comprehensively” at the 2018 engineering report into the building, as well as reviewing other documents.

“We’re now doing a really deep dive into the documentation, into the communications that had been had over the years with that particular building, and the other buildings for that matter, but also specifically the sister building,” he said.

But he reiterated that the rescue operation would be the top priority. “Buildings don’t fall down in America,” Burkett said on ABC News. “There was something obviously very, very wrong at this building, and we need to get to the bottom of it, but not today, not tomorrow and not for a long time, because our first priority and our only priority is to pull our residents out of that rubble.”

Levine Cava said she would be “supportive” of anyone living in the sister block who wanted to evacuate and said building inspectors were being sent to do “a more detailed review of that building’s structure” after an initial building inspection did not find any cause for concern.

Burkett said the city would provide resources to any of those residents who wanted to relocate.

Earlier in the week, US president Joe Biden ordered federal assistance to supplement state and local response efforts, and declared a state of emergency. The emergency action authorises the Department of Homeland Security’s Federal Emergency Management Agency to co-ordinate “all disaster relief efforts”, a White House statement said on Friday.

*This story has been amended after officials identified the remains of four people who had been unaccounted for