The ongoing investigation has so far proven that the munition did not go off target, but rather hit the school because old intelligence showed it to be a military target, four sources familiar with the preliminary findings said.

Witnesses and an Iranian Education Ministry official said previously that the school was located on a compound that was a base for the Islamic Revolutionary Guard Corps until about 15 years ago.

The two sources said the Defense Intelligence Agency was one of the agencies responsible for providing targeting information that could have led to the school strike, though other sources likely contributed to verifying the target, including intelligence from U.S. allies.

President Donald Trump has previously denied that the U.S. was behind the strike, and suggested without evidence that Iran or “other countries” could be responsible.

“In my opinion, based on what I’ve seen, that was done by Iran,” he told reporters onboard Air Force One on Sunday.

Missile fragments purported by Iranian state media to have struck the school bear the markings of an American Tomahawk missile, according to experts who reviewed the imagery, obtained by NBC News and others. The U.S. is the only country currently involved in the conflict that uses Tomahawk missiles.

“Whatever the report shows, I’m willing to live with that report,” Trump said Monday when pressed on the strike.

A group of United Nations experts warned last week that an “attack on a functioning school during class hours raises the most serious concerns under international law,” noting that “intentional attacks on educational buildings” are considered war crimes.