Israeli defense officials say an investigation by the military has concluded that an errant Israeli mortar shell was behind a deadly strike near a U.N. school in Gaza last week.

Gaza health officials said at the time that 39 people were killed in Tuesday's strike. Israel said its forces were returning fire toward Hamas militants operating near the school.

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The Israeli investigation concludes that troops fired three mortar shells at Hamas militants who had just launched a rocket.

Two shells hit the target. One missed by about 30 yards, causing the civilian casualties. The Israeli military believes the number of casualties was inflated by Hamas.

The defense officials spoke Sunday on condition of anonymity because the investigation has not been made public.