The military said on Wednesday that old Israeli ordnance could have killed the eight Palestinians last Friday, after its previous denial of responsibility was received with scepticism by the United Nations and elsewhere.
General Meir Klifi, who is leading the Israeli military investigation, said an examination of shrapnel removed from one of the casualties proved that it did not come from a 155mm shell - the type his army was firing at the Gaza Strip last Friday.
But he told Israeli radio: "It could be from another kind of munition that we have used in the past."
He said, however, that his investigation was incomplete so his findings were not conclusive.
This was a retreat from his position on Tuesday night when he appeared at a news conference with General Dan Halutz, the chief of staff, who said "we do not bear responsibility" for the deaths.
Amir Peretz, the Israeli defence minister, also said: "We have enough evidence that back our big suspicions that the attempt to present this as an Israeli incident is simply untrue."
Palestinians have asked for a UN investigation into the incident.
Human Rights Watch, a US-based group, has already inspected the site of the explosion.
Marc Garlasco, a former Pentagon adviser who is now a military analyst with the group, said: "It is my contention that the most likely scenario is that Israeli shelling hit the area."
Of Israeli suggestions that the blast was caused by a Palestinian mine, he said: "This is patently not the case." He told AFP: "We are very certain that it is a 155mm shell."
'Nothing to hide'
Kofi Annan, the UN secretary-general, called Israel's claims "odd".
Asked whether an international inquiry was needed, he said: "We would need both the Israelis and Palestinian authorities to co-operate with such an investigation."
An Israeli foreign ministry official said no request had been received from the UN to conduct an inquiry, but said Israel would co-operate if one were received. "We have nothing to hide," he said.