Conspiracy to assassinate the President was a federal crime under 18 U.S. Code ? 372. That gives the Feds the authority to seize evidence. The Feds asserted their investigative or prosecutive jurisdiction when they removed JFK's body from Dallas.
Talk about a circular argument.
Be specific. What statute gave the feds the authority to seize evidence from a state investigation? 18 U.S. Code ? 372 doesn't.