"Israel has dismissed continuing calls for an independent international inquiry into the beachfront explosion which killed seven members of a Palestinian family in Gaza last Friday after its own internal military investigation decided it was not responsible for the blast.

As the military investigation team insisted that artillery fire had stopped by the time the explosion occurred and suggested it had been caused by a bomb planted in the sand, Amir Peretz, the Defence Minister, declared: " The accumulating evidence proves that this incident was not due to Israeli forces."

But the official interpretation was strongly challenged by a former Pentagon battle damage expert who has surveyed the scene of the beach explosion. He said yesterday that "all the evidence points" to a 155mm Israeli land-based artillery shell as its cause.

Marc Garlasco, who worked in war zones including Iraq and Kosovo during his seven-year stint in the US Department of Defence, called for an independent investigation into the killings after concluding that shell fragments and shrapnel from the site, the size and distribution of the craters on the beach, and the type of injuries sustained by the victims made Israeli shelling easily the likeliest cause.

His assessment came as at least another seven civilians, including two children, as well as two Islamic Jihad militants, were killed in a double Israeli missile strike on a VW van in the densely populated Zeitoun district of Gaza City yesterday. The two children were hit at a nearby house by flying shrapnel and the civilian dead included three medical workers from a nearby children's hospital who rushed to help after hearing the first explosion.

Israel said the militants had been on their way to launch Katyusha rockets which have a much longer range than the Qassam rockets normally fired from Gaza into Israel. One of the two dead Islamic Jihad militants was Hamoud Wadiya, described as the top rocket launcher in the faction. Mr Peretz said before the strike that Israel was resuming operations "to protect the citizens of Israel" after a pause caused by what he had acknowledged had been "the international storm" over the civilian deaths at the Beit Lahia beach last Friday.

The debate over the beach explosion is unlikely to die down however. Mr Garlasco who is now the senior military analyst for Human Rights Watch, said yesterday: "Of course I can't be completely conclusive but all the evidence points to its being a 155mm Israeli shell which killed the Palestinians on the beach."

Mr Garlasco said that most of the serious injuries of the victims in the Gaza hospitals that he had visited were to the torsos and heads, which were inconsistent with a land mine or of a bomb embedded in the sand. "If this had been a landmine I would have expected to see serious leg injuries," he said. Mr Garlasco said that while he could not rule out the theoretical possibility that Palestinian militants had rigged up an unexploded 155mm shell to make an explosive device of their own, that too would have normally produced many more severe leg injuries.

Mr Garlasco produced a four to five-inch, mainly blackened shell fragment which he collected about 100 yards from the scene of the explosion and in which the figures 55 and the letters "mm" are clearly discernible. While acknowledging that this was not itself definite proof that the shell had killed the Palestinians he said some fragments and shrapnel which the Palestinian police explosives department say they took from the scene where the victims were killed were definitely from a 155mm shell.

Mr Garlasco who accompanied a small group of journalists to the Beit Lahia beach, pointed to three separate craters, each covered in a whitish powder, which he said were fresh, one of which was at the spot where witnesses agree the fatal blast occurred, and the two others separated it from it by about 120 and 250 yards. Mr Garlasco added: "It would be a really ridiculous coincidence if there is active shelling and then suddenly an IED [improvised explosive device] goes off."

The military have admitted firing earlier in the area but now say that the explosion occurred between 4.47 and 5.10pm, when it says firing had stopped. An ambulance driver from the nearby al-Awda hospital, Khaled Abu Sada, said that he first took a call about the emergency at 4.50pm.

The military did not explicitly repeat claims in earlier leaks that Hamas had planted the device or say whether it was a dud shell. It says that shrapnel taken from the bodies of victims being treated in Israeli hospitals was not from a 150mm shell. But Mr Garlasco said that copper-lined shrapnel taken from two injured girls who had been in a car at the time of the blast and from the car itself were consistent with such a shell fired by a M109 howitzer.

Mr Garlasco ruled out the possibility that the shells were naval, as originally thought, on the grounds that they were too large to be fired from Israeli navy coastal vessels.