In my opinion, BK was POI #1 because he was the owner of a white Elantra with no front plate, lived nearby, and could not be excluded from DM's description. LE Moscow stated they had applied for, I think 55 warrants. IMO, that must have included AT&T in order to get cell phone ping data.
When BK left WA, LE likely, IMO, did not yet have enough evidence for probable cause for search and arrest; they were still methodically building the evidence. But what they wanted MOST, was access to the car, because it is thought that the killer got in the car immediately, and sped away. The car would be the best chance of finding victim DNA, or other evidence linked to the victims. This may be why he was allegedly followed to PA. I imagine the car itself was under surveillance, too, while LE was making sure their evidence was comprehensive enough for judges in WA and PA to approve warrants and arrest. LE dismantled the car looking for evidence from the victim's, their home, possessions, Murphy hair, anything that could link that car to the crime beyond it's proximity during the time the crime was thought to have occurred. So, we wait.