Il clan della Arroyo.

How was such an attack possible?
There are three reasons. First, the culture of impunity wherever President Gloria Macapagal Arroyo is involved. Second, the culture of rido, or clan feuds, in Mindanao, and third, defects in the Filipino judicial system.
That is a sufficient explanation of the massacre: The Ampatuans assumed they could get away with it, given the 16 leadership positions they hold on the huge island, including two governorships.

(Philippine massacre facilitated by confluence of factors)