The shootings started on Wednesday evening (the window shot at Michael's, the murder in the supermarket parking lot), followed by the four murders on Thursday morning, all in Montgomery County.
The DOJ arrested the Oregon cell members on Friday morning, and held a press conference to announce it on Friday afternoon, so that puts the cart before the horse for your theory; although it is a bizarre coincidence that Moose is the former Portland police chief.
Were the arrests completely secret? Or did they have any inkling that they were coming?