the Capitol riot? Oddly enough the blame on Trump isn't in the report, but it was politics per usual that caused a lack of security. It seems like Pelosi and Bowser should be the ones under investigation, since they did not want a heavy security presence.
https://www.cbsnews.com/news/capitol-police-riot-report-failures-january-6/
The request for more security that was denied
https://www.npr.org/2021/01/11/955548910/ex-capitol-police-chief-rebuffs-claims-national-guard-was-never-called-during-ri