“The Constitution does not create a general Federal police power.”
Abraham Lincoln did...or if you want to go back even further so did George Washington for the Whiskey Rebellion.
In both examples, the President used his obligation to take care that the laws be faithfully executed, after it became clear that "the ordinary course of judicial proceedings or the powers invested in the US Marshals" were inadequate to deal with the situation.
Murder is not a Federal crime, even if it's done with an AR-15.
Murder is a Florida offense, the courts are open in Florida, and unless I am mistaken the ordinary course of judicial proceedings still sends murderers in Florida to the electric chair, so I stand on my view that this is not an FBI matter.