Champlain Tower South -- a 12-story residential condo in the beachside town of Surfside, about 6 miles north of Miami Beach -- has 136 units. Fifty- 55 collapsed, officials said.
During an evening briefing on Thursday, officials said 102 people have been identified and declared safe.
"The building has literally pancaked," Mayor Charles W. Burkett said during a morning press conference.
Burkett said at least 10 people, including the person who died, were treated for injuries on-site. One person was taken to the hospital in critical condition. He added that 15 families walked out of the building on their own.
The mayor said the collapse happened around 1:03 a.m. -- meaning the building was likely filled with sleeping residents -- and fears the death toll could rise.
"It's heartbreaking because it doesn't seem to me ... that we will find people alive," Burkett said.
Florida Gov. Ron DeSantis called the incident a "tragic, tragic situation."
"We'll hope for the best in terms of additional recoveries, but we are bracing for some bad news just given the destruction we are seeing," he said during a press conference.
It's unclear how many people remain unaccounted for, but witnesses told Miami ABC affiliate WPLG that more people are still trapped inside. Multiple local officials tell ABC News there could be as many as 99 people unaccounted for based on the number of people in the building.
The Paraguayan president's sister-in-law and her family are among the missing, the country's foreign ministry confirmed. The first lady's sister Sophia Lopez Moreira, her husband Luis Pettengill, their three unidentified children and nanny Lady Luna Villalba were in the building at the time of the collapse, officials said.
A massive search and rescue operation was underway into the morning, as crews carefully checked the wreckage and remaining structure for survivors. Footage from the scene showed firefighters pulling a boy from the rubble alive and rescuing other people from balconies.
The Miami-Dade Fire Rescue said it has sent more than 80 units to the scene. Multiple police and fire departments from across Miami-Dade County were deployed to assist, according to the Miami Beach Police Department. People in neighboring buildings have been evacuated to a nearby recreational center, one witness told ABC News.
MBPD and @MiamiBeachFire are assisting the Town of Surfside at a partial building collapse located at 8777 Collins Avenue in Surfside, Florida. Multiple police and fire agencies from across Miami-Dade are assisting. Please follow @MiamiDadeFire for updated information. pic.twitter.com/8tORIfZfjY— Miami Beach Police (@MiamiBeachPD) June 24, 2021
Officials remain concerned about a secondary collapse as search and rescue efforts are underway. A storm also hit the area, further complicating rescue efforts.
It was unknown what caused the collapse. Burkett said the building was built in the 1980s and characterized a collapse like this as "less likely than a lightning strike."
Witnesses said construction was ongoing on Champlain Tower South's roof, but Burkett said he does not see how that caused the wreck.
The collapse appeared to affect one leg of the L-shaped tower. Piles of rubble and debris surrounded the area just outside the building, and cars up to two blocks away were coated with a light layer of dust from the debris.
The seaside condo development was built in 1981 in the southeast corner of Surfside. It had a few two-bedroom units currently on the market, with asking prices of $600,000 to $700,000.
The area has a mix of new and old apartments, houses, condominiums and hotels, with restaurants and stores serving an international combination of residents and tourists. The community provides a stark contrast from bustle and glitz of nearby South Beach with a slower-paced neighborhood feel.
ABC Owned Television Stations and the Associated Press contributed to this report.