From the story, there is not one single artilleryman in the world who cannot make a guess as to what actually happened. It sure wasn't a Qassam that killed these people. Olmert states the IDF is the most moral army in the world. The most moral army in the world doesn't try to cover up this mistake and an army that cares about civilians doesn't have such a high civilian/combatant kill ratio.
Yemen's Houthi rebels commit to peace plan in letter to UN chief, BBC reports (Haaretz)
from the article: Peretz: Gaza beach blast may have internal Palestinian cause