An IDF committee set up to probe the cause of an explosion that killed seven Palestinian civilians on Friday in the Gaza Strip got to work on Sunday, with senior officers saying they had received new information indicating that the blast was caused by an "internal Palestinian incident" and was not related to Israeli artillery fire. A Palestinian bomb planted on the beach or a Kassam rocket that went astray, senior officers said might have been the cause of the explosion. The officers said that Navy and IAF fire had been ruled out, and while the landing spot of one of six artillery shells fired close to the time of the incident had yet to be found, it was unlikely that the "missing shell" was the cause of the blast. The IDF said the timeline of the incident and the artillery fire did not match. In addition, the closest any one of the six shells landed to the beach was 200-250 meters away, while a shell usually only strayed from its target a maximum of 30 meters. OC Southern Command Maj.-Gen. Yoav Galant said the army had proof its cannons were not responsible for the blast since the IDF fire was suspended 15 minutes before the explosion at the beach. But other officers spoke more cautiously, claiming it was still too early to vindicate Israel in the deaths of the innocent Palestinians. Hamas operatives, the army revealed on Sunday, arrived at the scene of the blast moments after the explosion and were spotted collecting shrapnel. "It might be that they were trying to conceal evidence that would have put the blame on them," an IDF source explained. Meanwhile Sunday, at the weekly cabinet meeting, Prime Minister Ehud Olmert strenuously defended the IDF actions in the Gaza Strip. "I reject outright any and all attempts to impugn the morality of the IDF," Olmert said. "The IDF is the most moral military in the world; there has never been - and there isn't now - a policy of attacking civilians. I support the IDF commanders and soldiers who are working to halt the attacks on Israeli citizens in the southern part of the country." Olmert stressed that "it has never been Israel's policy to attack innocent civilians. At the same time, we will continue to act with full force against the firing of Kassam rockets and will not refrain from operations that can foil rocket firings anywhere and in any situation, as necessary." Herb Keinon contributed to this report.