Maybe--or maybe they really do think Ricci was a big part of it, or maybe even think he did it alone.
Maybe they are getting lazy and have just decided, well, the main guy is gone, so we don't need to do more. Maybe the FBI, on the other hand, has more funds and/or manpower available, and is raring to go.
This is not to say that SLCPD are necessarily correct if they are concluding that Ricci was it. But it does seem as though they think that.
Devil, you might as well give Ricci as rest and get on with really trying to figure out how to solve this case.