Anderson is in charge of the FBI investigation instead of Ward

At the very beginning of the movie when they want to speak with the sheriff, Anderson threatens Pell that he will kick the door down if the sheriff is not out there in a minute.

What if Anderson is in charge all along? 1964, Lyndon Johnson is President and he's pushing civil rights legislation through Congress. He's just gotten news of some fuckup in Viet Nam and then the news of the missing civil rights workers. Fully animated and pissed as hell, he gets Hoover on the phone and tells him "I want this shit in Mississippi cleaned up, I don't care if you have to burn every bed sheet down there, but I want these shit kickers taken down about 3 pegs, get it done."

Hoover hates King and is about as racist as anyone, but sensing this is a situation he needs to tame he puts known tough-guy agents on the job. He taps Anderson, as a former Mississippi sheriff, as suitable and tells him to organize a group of agents known for their street toughness to go down there and sort this out, giving them wide latitude to "get results."

How does it turn out with a smaller contingent of hard guys?