No, but the head honcho of the FBI determines how hard to push an investigation. Reportedly, Comey had requested a few days ago additional money and agents to investigate people in the Trump campaign as it pertains to their Russian activities. I suspect that whoever Trump nominates to replace Comey will be somebody who Trumps believes will be loyal to him and will do his biding. Bottom line: Trump fired Comey because he didn't like the way Comey was taking the investigation. Even IF that is not the case, and I don't believe it is, the appearance that that was the motivation is very real.