June 23 (UPI) — The June 2021 partial collapse of a Miami Beach oceanfront condominium tower that killed nearly 100 people began three weeks before the building completely failed, federal investigators announced Monday.

The four-decade-old Champlain Towers South condo in Surfside, Fla., collapsed June 24, 2021, prompting the deployment of first responders to scour the rubble for survivors. In total, 98 people were killed and many others were injured, making it one of the deadliest structural disasters in U.S. history.

The National Institute of Standards and Technology, which launched an investigation into the incident within days of the collapse, said Monday that it determined the collapse began in early June 2021, when two connections between garage columns and the building’s pool deck failed.

“These initial column failures caused cracks to grow and loads to redistribute in the pool deck over the next three weeks, resulting in the transfer of their loads to adjacent slab-column connections that were not strong enough to support them,” NIST said in a statement.

“This led to the larger catastrophic collapse on June 24.”

The 12-story Champlain Towers South building was constructed in 1981 under required codes and standards intended to ensure builders could support more load than they were expected to bear. But, Judith Mitrani-Reiser, who was a co-lead on the investigation, said the tower’s “margins against failure were not too narrow from the start.”

The investigators believe that the failure of the two connections then spread to other elements of the pool deck and street-level parking structure before unseating the southern edge of the pool deck slab from a supporting wall.

When the pool slab eventually broke away, it damaged two connections supporting that part of the tower, causing the failure to travel through the middle of the tower.

The investigators also found that loads added to the structure over the building’s lifetime, such as pool deck modifications, further narrowed the margins against failure on top of long-term degradation caused by corrosion.

The investigation team said it is now working on compiling a final report that will include recommendations for changes to standards, codes and practices, among other suggestions.

The announcement came after NIST in September released an update into its investigation stating that the condo had shown visible signs of structural strain weeks before the collapse, starting with the pool deck.