Bank of Montreal (BMO) said Wednesday it has concluded its acquisition of Marshall & Ilsley (M&I), becoming the 12th largest commercial bank in the United States. The $4.1 billion stock transaction, first announced in December, allowed the Canadian bank to double its stake in the US market, where it has been active since its 1984 purchase of Harris Bank, the third largest in Chicago. The merged banks, known as BMO Harris Bank and based in Chicago, will operate 700 branches and some 1,350 ATMs in six states in the US Midwest as well as Arizona and Florida.