Well, they need not go far. They can ask Sham Ampatuan (Zaldy Ampatuan's son?) He has blatantly published pictures of himself showing him test firing these weapons on his public Friendster profile. I am publishing the pictures here before he decides to go private and/or delete the pictures!
These pictures should help them establish ownership of the huge arms cache. They may also contain pictures of possible suspects who might have participated in the actual massacre!