What we know about the Miami building collapse in Surfside, Florida

Resident Barry Cohen and his wife were rescued from the building.

"At first it sounded like a flash of lightning or thunder," Mr Cohen, a former vice-mayor of Surfside, told reporters at the scene.

"But then it just kept on — steadily for at least 15 to 30 seconds — it just kept on going and going and going."

By early evening on Thursday, rescuers were still at the scene searching through the rubble, with about 100 people thought to be still missing. 

Here's what we know so far about the Miami building collapse.

What's happening now?

Florida chief financial officer Jimmy Patronis, the state's fire marshall, said teams of 10 to 12 rescuers were entering the rubble at a time with dogs and other equipment. 

They were working until they tired from the heavy lifting, then making way for a new team, he said.

"They're not going to stop just because of nightfall," Mr Patronis told Miami television station WPLG.

"They just may have a different path they pursue."

Miami-Dade Fire and Rescue assistant chief Raide Jadallah said they had been working their way through the rubble slowly and methodically.

"You see that every time there's a shift in the rubble, we have additional rubble that shifts on us," he said.

He said they had put out a small fire.

"We did receive sounds, not necessarily people talking, but sounds, what sounds like people banging, well, not people, but sounds of a possibility of a banging," he said.

"Short of that, we haven't heard any voices coming from the pile."

Florida Governor Ron DeSantis made a state of emergency declaration, meaning federal authorities could provide assistance for the families affected.

Miami-Dade County Mayor Daniella Levine Cava said she got a call from US President Joe Biden, who promised to provide any federal aid that was requested.

Hotels already had opened to some displaced residents, the mayor said, and deliveries of food and medicine were being hastily arranged.

At an evacuation site set up in a nearby community centre, people who live in buildings neighbouring the collapse gathered after being told to flee. Some wept. Some were still dressed in pyjamas. Some children tried to sleep on mats spread on the floor.

How many people died or were injured?

Only one person has been confirmed dead so far, but with about 100 people thought to be missing that number is expected to rise.

And officials aren't even sure how many people were in the building when it collapsed.

More than 100 people have been accounted for.

Rescuers pulled at least 35 people from the wreckage by mid-morning.

Among them was a mother and child, but the woman’s leg had to be amputated to remove her from the rubble.

Video also showed fire crews removing a boy from the wreckage.

Miami-Dade Police Director Freddy Ramirez said the numbers of known casualties and people missing were likely to fluctuate.

"I don't want to set false expectations," he said.

"This is a very tragic situation for those families and for the community."

Who was in the building?

The Champlain Towers South had more than 130 units, about 80 of which were occupied. 

Surfside Mayor Charles Burkett said the tower had a mix of seasonal and year-round residents, and while the building kept a log of guests, it did not keep track of when owners were in residence.

Joseph Waks, an Australian who has been living in Surfside for 12 years, said an Australian family lived in the building, but he had not been able to locate them.

A total of 22 South Americans were missing in the collapse — nine from Argentina, six from Paraguay, four from Venezuela and three from Uruguay, according to officials in those countries.

Among the Paraguayans were Sophia López Moreira, the sister of first lady Silvana Abdo, her husband Luis Pettengill, her three children and the family's assistant, the country's authorities said.

President Mario Abdo's activities were cancelled for Thursday and Friday so that he could be with his wife while she waited for word on the fate of her sister and her family, Paraguay's government announced.

Paraguay’s presidential palace told AP the first lady was planning to travel to Miami, Florida, on Thursday on a private plane. 

Director Ramirez told reporters that 99 people were unaccounted for and that 53 others whose whereabouts were initially unknown had since been located.

Why did the Miami building collapse happen?

Authorities have not yet said what may have caused the collapse.

The seaside apartment development was built in 1981 in the south-east corner of Surfside, a seaside enclave of 5,700 residents on a barrier island across Biscayne Bay from the main city of Miami.

Florida professor Shimon Wdowinski told CNN a study he had co-authored found the building had been subsiding about 2 millimeters a year from 1993 to 1999.

Professor Wdowinski said such subsidence probably did not cause the collapse but could have been a contributing factor. 

On video footage captured from nearby, the centre of the building appeared to fall first, with a section nearest the ocean teetering and coming down seconds later as a huge dust cloud swallowed the neighbourhood.

Work was being done on the building's roof, but Mr Burkett said he did not see how that could have been the cause.

Surfside city commissioner Eliana Salzhauer told WPLG the building's county-mandated 40-year recertification process was ongoing.

However, Ms Salzhauer said the process was believed to be proceeding without difficulty.

A building next door was also under construction. 

"I want to know why this happened," Ms Salzhauer said.

"And can it happen again? Are any other of our buildings in town in jeopardy?"

ABC/wires