Four people are now known to have died in the collapse of an oceanfront apartment building near Miami Beach, officials said Friday, while the number of unaccounted for has risen to 159 -- fueling fears of a much higher death toll.

An unknown number of residents are feared to have been asleep in the 12-story tower in the town of Surfside, when one of its wings was reduced early Thursday to a gigantic pile of debris.

"We do have 120 people now accounted for, which is very, very good news. But our unaccounted for number has gone up to 159," Miami-Dade County mayor Daniella Levine Cava told a news conference.

"In addition, we can tragically report the death count is now four," she said, adding that the numbers were "very fluid."

Rescue teams with sniffer dogs again worked through the night despite heavy rain -- clinging to the diminishing chance of finding additional survivors.

Their efforts were illuminated by lights shining on the debris, with the recovered bodies put into yellow bags and transported away as homicide detectives worked to confirm the identities of the victims.

Firefighters were watering the north side of the still-standing building after a fire broke out, and the smell of rubber and plastic hung in the air amid patches of heavy rain.

"We will continue search and rescue because we still have hope that we will find people alive," said Levine Cava -- who described the dedication of the dozens of rescuers on site as "incredibly moving."

"They are totally, totally motivated to find people. They have to be pulled off the shift."

President Joe Biden declared an emergency early Friday in response to the disaster, ordering federal assistance for the relief effort.

At a Surfside community center Thursday night, relatives of the missing wept as they waited for news. Occupants who were lucky enough to have been away when disaster struck pondered sudden homelessness.

Erick de Moura, 40, happened to spend Wednesday night at his girlfriend's house, so he was not present at the time of the collapse.

"I just came back and the scene is shocking," he said. "There is a lot of pain. I'm blessed that I am alive."

'We have hope'

The building was occupied by a mix of full-time and seasonal residents and renters, and officials have repeatedly stressed it is unclear how many people were actually inside at the time.

Around 55 apartments were affected by the collapse. Some residents were able to walk down the stairs to safety while others had to be rescued from their balconies.

Miami-Dade Fire Rescue assistant chief Ray Jadallah said search teams were guided by the sounds they detected "throughout the night."

"It's not specifically, you know, human sounds -- it could be, you know, tapping, it could be steel, you know, kind of twisting, it could be some of the debris kind of raining down," he explained to reporters.

"We have hope. And every time that we hear sound, we concentrate in that area."

Questions over building

While the reasons for the collapse remain unclear, the condition of the 40-year-old waterfront building is certain to face scrutiny.

Completed in 1981, Champlain Towers was due to be recertified this year in line with Miami-Dade county safety regulations, and was undergoing construction work on its roof as part of that process.

According to a study led by Florida International University environment professor Shimon Wdowinski, based on space-based radar data, the site showed signs of land subsidence in the mid-1990s.

"I don't know if the collapse was predictable. But we did detect that the building moved in the 1990s," Wdowinski told CNN Friday.

The professor said he preferred to describe what was happening as a "slow process" of settling, rather than sinking.

FIU cautioned in a post on its website that "land subsidence in and of itself likely would not cause a building's collapse."

And another of the university's experts, Atorod Azizinamini, chair of its department of civil and environmental engineering, said in an online video that it was too soon to speculate on a cause.

He said structural engineers would collect vast quantities of data on design plans and construction methods, take samples of steel and concrete, look at signs of corrosion, examine the foundation, and try to detect any unusual event before the collapse.

"Once we have all the information we can simulate exactly different scenarios, and we can pinpoint how the collapse took place," Azizinamini said. "Unfortunately that is not going to be happening in a matter of days, weeks."

"It's going to take some time."