Just because Russia has attacked and seized territry from its neighbors multiple times in the past 20 years and its government has repeatedly stated its desire to absorb more former Soviet states and bring former Warsaw Pact vassals back under its control, who could believe Russia is responsible?
Poland's government has accused Russian intelligence services of orchestrating the arson attack that destroyed the Marywilska 44 shopping center in Warsaw on May 12, 2024 . The claim comes after a year-long investigation into the blaze that gutted 1,400 shops and service outlets.
Official conclusion: In May 2025, Polish Prime Minister Donald Tusk publicly announced that Polish authorities had "evidence that the Russian special services committed a reprehensible act of sabotage".

https://www.bbc.com/news/articles/cgegj1rvq84o
I'd love to have a source for that.