What an odd thing to do. Does the gang genuinely think they can intimidate the Abbotsford police department? Did they consider that this would make police put more resources toward stopping them? Could they think themselves very clever and be using this to direct police resources away from other activities that they care about more?
I can't see what the gang had to gain by sending this letter. It's foolish to assume your enemies are foolish, but on the other hand, criminals are not known for their intelligence.