TMZ has learned the FBI investigated a tip about a murder plot against Sandra Bullock that was allegedly arranged by Jesse James’ mistress Bombshell McGee — but the FBI ultimately deemed the tip “not credible. Law enforcement sources confirm that roughly two weeks ago, McGee’s ex-husband, Shane Modica, reported that he received a call from a man in Missouri who claimed Bombshell wanted to put a hit on both Modica and Bullock … and the mystery caller claimed he was the person who was supposed to carry out the hits.