Federal investigators examining the final days at MF Global and how $1.6 billion went missing are poring over scores of wire transfers in and out of the brokerage firm, including the possible movement of $325 million that may have belonged to customers, according to people briefed on the matter. The suspicious transfer, which until now has not been made public, was first discovered in the early hours of Oct. 31, the day the firm filed for bankruptcy. Initially, the firm attributed a shortfall in customer money to an “accounting error,’’ records show. But after hours of searching, MF Global executives acknowledged to regulators huddled in the firm’s offices in Chicago that the shortfall was real - and may have been caused in part by the $325 million transfer, said one of the people briefed on the matter.