"Their whereabouts are known and they are safe," Cava said at a news conference at 4:30 p.m. ET.
Ninety-nine people are still missing after the building cracked and part of it collapsed in the middle of the night.
County police have confirmed that one person has died. An extensive and thorough search and rescue operation is underway.
Miami-Dade Fire Rescue released a video showing searchers in the flooded basement of the building.
#MDFR #TRT & #FLTF1 are working in the basement parking garage at Champlain Towers. Firefighters continue working on locating possible victims, while dealing with heavy damage and changing conditions in the parking garage. #SurfsideBuildingCollapse pic.twitter.com/qseknk0T8q— Miami-Dade Fire Rescue (@MiamiDadeFire) June 24, 2021
Deputy Fire and Rescue Chief Ray Giadalla said rescuers have shifted their efforts from searching above the rubble to searching below. He said the parking structure under the building opens up new pathways in the mass of debris.
Since firefighters arrived on the scene at 1:48 local time, 37 people have been extracted from the building alive.
Jadalla described the process as slow, dangerous and methodical, adding that as some firefighters push aside piles of rubble in search of victims, small fires break out.
Cava said federal help from the Federal Emergency Management Agency is on the way. The mayor said she signed the emergency order, which is now on Gov. Ron DeSantis' desk.
Crews are working with civil engineers to create new passageways into the collapsed building using sonar devices and search cameras. Officials said they heard "sounds" coming from inside, and possibly banging. But they did not hear any voices.
The collapse sent dozens of rescuers rushing to the beach.
Firefighters and emergency responders said about 55 apartments in the Champlain Towers South complex, a 40-year-old building with more than 136 apartments in all, were destroyed in the collapse.
Miami-Dade County Commissioner Sally Heyman told NPR that if someone is missing, it doesn't mean the person is still in the building - they could have been away from home for a variety of reasons, such as work or business trips.
Authorities have asked residents who are safely out of the building to file a welfare check form so crews don't have to search for them.
The collapse occurred around 1:30 am. A video obtained by local news channel WSVN shows that the building collapsed in 15 seconds, leaving behind tons of rubble and a huge column of dust and debris.
JUST IN: 7News has obtained surveillance video of the moment the Champlain Towers South Condo collapsed in Surfside early this morning.— WSVN 7 News (@wsvn) June 24, 2021
According to a fire official, 35 people were pulled from the collapsed building. Search and rescue efforts ongoing. https://t.co/Ac7KgnJOSO pic.twitter.com/oeczbumRG9
"It looks like a bomb went off," Surfside Mayor Charles Burkett said on NBC's Today show.
Burkett said rescuers turned to dogs to help search for survivors, but "unfortunately, the dogs were unharmed, and that's a big disappointment."
"Apparently, when the building collapsed, it collapsed, so you don't find or see many holes on the outside," the mayor said.
Video and photos show a scene reminiscent of a war zone: the entire east side of the complex is almost completely destroyed. Empty beds and twisted air conditioners hung from the rubble as rescuers scoured the rubble for survivors.
Family rescued by a partially collapsed staircase.
Among those who managed to escape was Albert Aguero, who was vacationing in the Champlain Towers South building with his wife and two teenage children.
"It woke me up," Aguero told NPR station WLRN. "I felt the whole room shaking. My wife jumped out of bed to check on the kids. They were fine. We looked out on the balcony. There was a cloud of smoke."
Agüero said the family was able to escape by jumping down the partially collapsed staircase.
Debris hangs from a partially collapsed building in Surfside, Florida, Thursday. A multi-story apartment building north of Miami Beach partially collapsed early Thursday, prompting extensive emergency work.
Chandan Khanna/AFP via Getty Image
"When we left the apartment, I looked to the left and the ceiling was completely collapsed in the apartment to our left. I looked ahead, where the elevators were, and there was just a shaft in a hole," he said.
By mid-morning, everyone remaining "in the undamaged part of the building" had been safely evacuated, Mayor Danielle Levin Cava told NPR.
"What a terrible tragedy," she said.
Levin Cava said it is too early for investigators to judge the cause of the building collapse, but said investigators are working at the scene along with engineers and other rescue workers.
Burkett said work was being done on the building's roof, "but you would never expect that to be the problem." The foundation, he said, "somehow got undermined and the whole thing collapsed."
Freddy Ramirez, director of the Miami-Dade Police Department, said his department is acting as lead agency in the investigation of the collapse and is working with the town of Surfside. The Miami-Dade Fire Department is leading emergency response efforts and has set up a family reunification center a half-mile from the building.
Burkett told NBC that 15 families have already been reunited at the city's community center and will soon be moved to hotels.
It's a developing story. Some of the details reported in the media may not be correct. We will be guided by reports from police and other authorities, credible media and reporters on the scene, and will update information as the situation develops.