Three weeks before the Champlain Towers South condominium partially collapsed — a 2021 tragedy that claimed 98 lives — the building showed signs of impending disaster, according to federal investigators.

The National Institute of Standards and Technology, in a report released on Monday, concluded a structural failure occurred at two crucial garage column connections below the building’s pool deck in early June of that year.

That failure then ushered the building into a slow-moving, and undetected, catastrophe over the course of 21 days.

This outcome confirms what engineers had suspected for years — even in the immediate wake of the incident, whose fifth anniversary is Wednesday.

During that weeks-long period, the 12-story building exhibited red flags that no one picked up on. Chief among them, a rise in water leaking through the garage in the hours leading up to the collapse. But even earlier, an apartment’s sliding glass door had popped out of its frame, according to the Wall Street Journal.

When the two columns failed, according to the investigators, the pool deck’s concrete slab began to warp and break during those 21 days — which shifted weight to nearby columns.

On June 24, the pool deck collapsed, which took part of the building with it.

However, the investigators revealed, the issues stem from missteps of decades past — mainly that the design didn’t meet codes and standards from the 1980s. What’s more, the completed work didn’t match the building’s original design, leading to weaknesses.

In the 1990s, dressing the pool deck with hefty planters and thick pavers added uncalculated weight to the slab. Subsequent years also saw the effects of corrosion, which all brought about a recipe for disaster.

The Champlain Towers South site is now home to the Delmore — a 12-story, 37-unit condo rising on the lot.

Sales launched there in January 2025 with prices from $15 million for furnished four- to five-bedroom flats — but, nearly 18 months on, not a single home has sold.

The developer, Dubai-based Damac Properties, bought the 1.8-acre parcel for $120 million in a 2022 court-ordered auction, where it was the sole bidder.