A partial building collapse in Iowa that killed three people in May was caused by the removal of brick and inadequate shoring of the 116-year-old structure, according to a report released by officials Thursday.

The 113-page investigative report by two engineering companies also blamed the collapse on an improper understanding of a structural bearing wall, inadequate oversight of repairs and a history of improper maintenance.

The city hired the engineering companies within days of the May 28 partial collapse of the apartment building in Davenport, which killed three residents and forced crews to amputate the leg of another resident to free her from rubble. The report was dated Aug. 15 and posted on the city's website Thursday.

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A city spokesperson did not immediately respond to a request for comment about the report.

The report found the root case of the collapse was the removal of brick during repairs in the three days before the collapse, which compromised the six-story building's west wall. The report said shoring installed on the wall was "grossly inadequate."

"Had a proper shoring and construction phasing plan been implemented during these repairs, the building would not have partially collapsed on May 28, 2023," the report said.

The site of an apartment building collapse that left three men dead is seen in Davenport, Iowa, Tuesday, May 30, 2023. (AP Photo/Erin Hooley)

Besides inadequate and improperly installed shoring, the report found that engineers and masonry contractors didn't realize the wall they were repairing was a structural bearing wall.

"As such, they underestimated the significance of the observable signs of distress in the wall, delayed necessary repair work, designed and installed a weaker replacement system, and removed significant portions of the wall without first installing adequate temporary shoring," the report said.

The report also cited inadequate construction documents that made it difficult for city inspectors to verify work completed, a lack of on-site oversight by a "qualified design professional" and a history of improper and inadequate repairs to the wall that ultimately collapsed.

OWNER OF COLLAPSED IOWA APARTMENT BUILDING BLAMES ENGINEERING COMPANY IN NEW LAWSUIT

The building collapse has led to numerous lawsuits filed by residents against the building owner, engineering company, city and others. Building owner Andrew Wold also has filed a lawsuit that blames an engineering company for not warning that the building was structurally unsound.

In the days after the collapse, residents and some relatives of those killed criticized city officials for their oversight of the building and emergency response. City documents showed a history of problems at the building but residents were not warned that the structure was potentially dangerous.

Davenport Mayor Mike Matson has called for an investigation into the collapse but also defended the city's actions, saying ""I don’t know that anyone can anticipate a building collapsing."

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The remains of the building were demolished in the weeks after the collapse. The downtown site is now bare ground.