A U.S. munition appears to have been responsible for a deadly missile strike on an Iranian elementary school that killed more than 170 people, according to preliminary findings from a U.S. investigation.

The strike occurred on the first day of armed conflict between the United States and Iran, hitting the school facility and resulting in significant civilian casualties, Iranian officials reported.

Two sources familiar with the investigation told NBC News that evidence increasingly points to U.S. responsibility for the attack. One U.S. official and another person with knowledge of the preliminary investigation findings both indicated that an American munition likely caused the strike.

The investigation into the incident remains ongoing, with U.S. officials working to determine the exact circumstances that led to the school being targeted. The preliminary assessment represents the current understanding of events based on available evidence.

The high civilian death toll at the elementary school has drawn significant attention as investigators work to piece together what occurred during the opening phase of the military engagement between the two countries.