Israel acknowledged Sunday that troops fired a mortar shell that hit the courtyard of a UN school in Gaza last week, but said aerial footage shows the yard was empty at the time 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 the town of Beit Hanoun last Thursday, killing 16 people and wounding dozens. It was one of the single deadliest incidents during three weeks of Israel-Hamas fighting.
The school was one of dozens used to house tens of thousands of Palestinians displaced by heavy fighting, especially in areas of Gaza bordering Israel.
The UN aid agency that operates the schools called for a full 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 Sunday.
He noted that when the UN agency attempted to conduct its own investigation, "the mission was aborted after a firing incident at the school." Gunness did not say who he believed was responsible for that fire.
He said the school had been clearly marked as a UN shelter, and that the IDF was aware of its location. On Thursday, the UN made numerous phone calls to the army to request a pause for the evacuation of civilians, but that the request was not granted.
Lt.-Col. Peter Lerner, an IDF spokesman, said Sunday that a military probe shows that "a single errant mortar landed in the courtyard."
It is "extremely unlikely that anybody was killed as a result of this mortar," he said. Lerner pointed to aerial footage released Sunday that 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.
By the time the photographer arrived at the school, the wounded had already been taken to the hospital. At the hospital they told an AP reporter that they had been wounded at the UN school.
Lerner raised the possibility that shrapnel from the shell might have wounded some at the school. He also offered other scenarios - that the wounded were "brought to the compound after injury" or were caught in a crossfire between Israeli troops and Gaza militants.
Saed al-Saoudi, the commander of the Civil Defense in Gaza, said Sunday that "all the testimonies of the wounded, witnesses, paramedics and doctors confirm that the Israeli shells are the cause of this massacre."