ok, here's my speculation -
They had the dna of the killer, a possible name and the car id, BUT they hadn't gotten an actual dna sample from BK yet. Or maybe they had, but it wasnt done being tested yet. There could be no arrest until they were certain they had the right guy. Maybe LE was just told to keep an eye on him and report back what they found out. (working for WSU or where they were headed kind of stuff.) It would be terrible if they arrested him, made a big scene and it turned out the dna didn't match.
Just thinking...JMO....