CNN  — 

Dozens of condominiums in a Surfside, Florida, residential tower were destroyed when part of the building collapsed early Thursday.

At least four people were killed and 159 people are unaccounted for, Miami-Dade County officials said Friday morning.

A search and rescue operation is underway after one side of the building fell to the ground for reasons that remain under investigation.

Here is what we know about the building and surrounding area:

Address: 8777 Collins Avenue

Built: 1981

Stories: 13 (the building stands about 160 feet tall)

Residences: The oceanfront building contained 136 one-, two- and three-bedroom units, according to real estate records. Most are between 1,200 and 2,243 square feet.

Joe Raedle/Getty Images & Nearmap

Price: Most condos range from $295,000 (for a one-bedroom in March 2020) to $980,000 (for a three-bedroom a year later), real estate records indicate. A four-bedroom penthouse spanning more than 4,500 square feet sold for almost $2.9 million earlier this year.

Other amenities: In addition to housing, the building offered terraces, a club room, tennis courts, fitness center, sauna, private beach, heated swimming pool, underground parking deck and 24-hour security.

Inspection: A study last year showed signs the building was sinking at a rate of about 2 millimeters a year between 1993 and 1999, said Shimon Wdowinski, a professor with Florida International University’s Institute of Environment.

Sinking was unique to the area of Champlain Towers South, not surrounding buildings, he said. Buildings in nearby western Miami Beach, which was built on reclaimed wetlands, were moving at higher rates, “so we didn’t think it was something unusual,” he said.

While the sinking alone likely would not cause the tower’s collapse, it could be a contributing factor because “if one part of the building moves with respect to the other, that could cause some tension and cracks,” Wdowinski said.

Over the last several months, the building has undergone “thorough engineering inspections” in preparation for its 40-year certification, said Kenneth Direktor, an attorney for the condo tower residents’ association, and “nothing like this was foreseeable.”

Ongoing construction: At the time of the collapse, the building was undergoing work on its concrete roof, Burkett said, but it’s unclear whether the work was a factor in the collapse. Work was being done to meet “40-year standards,” a strengthening of the building code – regarding updates and improvements – enacted following Hurricane Andrew in 1992, Miami-Dade County Commissioner Sally Heyman said.

An engineer who inspected the building determined it needed repairs, but the only repair underway was the replacement of the roof, Direktor said. It was the attorney’s understanding the engineer was helping the city investigate the collapse, he said.