From ABC News:

In the Air Force investigation of the incident, the final report concluded the two pilots should have held fire and left the area until they were assured no friendly forces were in the area. And controllers in an AWACS plane overhead can be heard in the cockpit tape telling Schmidt to hold his fire.

But seconds later, Schmidt apparently felt he had to take action. "I've got some men on a road, and it looks like a piece of artillery firing at us," he can be heard saying on the cockpit tape. "I am rolling in in self-defense." The AWACS acknowleged the transmission and about 30 seconds later, Schmidt fired his bombs.