The DOJ conducted an internal investigation after pressure from congress on Operation Chokepoint. Not surprisingly, the Obama administration didn't allow an independent investigation, and the DOJ’s Office of Professional Responsibility decided that what they did (restricting banking services to unfavored, but legal businesses including gun shops) was just fine.
Main article: Justice Dept. Clears Itself of Wrongdoing, Says It Didn't Target Gun Companies in Controversial Operation Chokepoint
45pg DOJ response letter: https://www.manatt.com/uploadedFiles/Content/4_News_and_Events/Newsletters/BankingLaw@manatt/OPRInquiryRegardingOperationChokePoint.pdf
Main article: Justice Dept. Clears Itself of Wrongdoing, Says It Didn't Target Gun Companies in Controversial Operation Chokepoint
45pg DOJ response letter: https://www.manatt.com/uploadedFiles/Content/4_News_and_Events/Newsletters/BankingLaw@manatt/OPRInquiryRegardingOperationChokePoint.pdf