Israel has acknowledged that troops fired a mortar shell that hit the courtyard of a UN school in Gaza last week, but said aerial footage showed the yard was empty and that the shell could not have killed anyone.
The shell was not fired at the school intentionally, an army spokesman said.
Palestinian officials have said three Israeli tank shells hit the school in Beit Hanoun last Thursday, killing 16 people and wounding dozens. If true, this would be one of the single deadliest incidents during three weeks of Israel-Hamas fighting.
The UN aid agency that operates the schools called for an investigation. "It is important in a case like this where a UN school in which hundreds of people took refuge is hit in this way, that there should be full transparency and accountability," Chris Gunness, a spokesman for the United Nations Relief and Works Agency, said.
Gunness said that when the UN agency attempted to conduct its own investigation, "the mission was aborted after a firing incident at the school". He did not say who he believed was responsible for the incident.
He said the school had been clearly marked as a UN shelter, and that the Israeli military was aware of its location.
Lieutenant Colonel Peter Lerner, an Israeli army spokesman, said that "a single errant mortar landed in the courtyard".
It was "extremely unlikely that anybody was killed as a result of this mortar," he said. Lerner pointed to aerial footage he said showed the courtyard was empty at the time the mortar was fired.
AP photos from the scene shortly after the incident showed large spots of blood on the edges of the courtyard and people's belongings strewn about.
Lerner said shrapnel from the shell might have wounded some at the school. He also suggested other scenarios - that wounded were "brought to the compound after injury" or were caught in crossfire.