Hindsight is 20/20, but the cops did have two opportunities to nail the snipers:
1. The tipline received an anonymous call suggesting John Malvo be checked out. The tip was disregarded due to his age.
2. There was a witness who described a brown or burgundy Caprice leaving one of the crime scenes, but for some reason that vehicle never received the notoriety of the "white van/box truck".
In context, both may have been a bit thin at the time.