A preliminary investigation into the fatal shooting by the Israel Defense Forces into a United Nations building in northern Gaza on Tuesday reveals the Israeli troops firing on the building missed their targets by some 30 meters.

Hamas is claiming the mortar fire killed 42 people and left dozens wounded, but senior IDF officers say the figures are dubious and that Hamas is inflating the numbers.

The probe, which was conducted by the Paratrooper Brigade whose troops were responsible for the area, found that the army's location system to pinpoint launch sites indicated that militants had launched a Qassam rocket into Israel from within a yard adjacent to the courtyard of the UN building.

The troops had intended to launch a smart missile to take out the Palestinian launch team but a technical malfunction made this impossible, according to the probe. The commanders of the force instead decided to fire on the Qassam team with mortar shells equipped with a Global Positioning System for accurate fire.

However, the GPS element has an error margin of 30 meters and one of the three rounds fired by the paratrooper force slammed into the building owned by the United Nations Relief and Works Agency for Palestine Refugees in the Near East, or UNRWA.

Two of the rounds hit the yard used to launch rockets into Israel, killing two members of Hamas' military wing who probably belonged to the squad that fired the rockets.

Nonetheless, in discussing the incident with Haaretz, some IDF officers say the force should have refrained from using mortar rounds and relied instead on more accurate fire. Military sources said the UNRWA building was marked on the maps of forces operating in the area.

Other officers said they found the death toll published by Hamas grossly exaggerated, pointing out that a week ago only three IDF soldiers were killed when a tank fired two rounds - which have a much larger impact than mortar rounds - into a building which was occupied by 50 IDF soldiers.

Officers interviewed for this article pointed out that Hezbollah resorted to similar inflation tactics after an IDF bomb landed on a UN post in Qana in southern Lebanon in 2006.