Once the FBI acquired key material evidence that evening he fell in line.
Yes, You're right.... Day did not disassemble the Carcano and discover a palm print on the 5/8" diameter metal barrel,but after Henry wade had told the whole wide world that they had found Lee Oswald's prints on the Carcano Day was obligated ( He wanted to continue receiving a nice paycheck, and enjoying the green grass from above the surface) to swear that he had found the print on the metal barrel of the rifle.
PS... Personally I doubt that Detective Day had the mechanical ability to disassemble the Carcano .......