The Israeli military on Saturday acknowledged that the initial accounts from troops involved in the killing last month of 15 people in southern Gaza — who the United Nations said were paramedics and rescue workers — had been partially “mistaken.” The assessment, which was shared in a briefing with reporters by an Israeli military official, came the day after a video obtained by The New York Times appeared to contradict the military’s earlier version of events. The official spoke on the condition of anonymity under army rules. The Israeli military official said the internal investigation of the attack, which has drawn international scrutiny and condemnation, is ongoing. Briefing reporters on Saturday night on the military’s initial findings, the official said forces from a reserve infantry brigade had been lying in ambush along a road to the north of the Gazan city of Rafah in the pre-dawn hours of March 23 and, at 4 a.m., had killed what he described as two Hamas security personnel and detained a third one.