The Safety Investigation Authority OTK said in a release Friday that initial findings show extensive rusting of the load-bearing steel frame supporting the riding hall.
The roof of the Laukaa riding school collapsed suddenly on February 13, killing one child and injuring four other people, including one adult.
The Authority said that the bolted connections also showed signs of damage, but that it is still looking into the role of this element in the accident.
OTK said that the rust damage may have come from inadequate surface treatment of the metalwork. The Authority added that this may have weakened the structure over time.
OTK has called for a wider inspection sweep of structures like the Laukaa riding hall, and has warned that other buildings in Äetsä, Pöytyä and Östersundom may be at risk.
The full investigation will be complete by the end of the year, but key findings will be released by the summer.