Sometime before 1 BBY several Sulonese farmers and colonists, complained about Imperial policies and eventually publicly gathered in the city of Barons Hed for a nonviolent demonstration. Imperial forces reacted but not before they scramled.
To avoid embarrassment, the local commandant resolved to the recorded holovids (and the collaboration of a traitor) to identify and hunt them.
All Terrain Armored Transports had the information from Imperial Intelligence that the colony had about four hundred Rebels and the walker operators sought to kill as many as possible; one walker on purpose targeted a dormitory, and twenty to thirty civilians escaped the rubble to declare their surrender, only to be targeted and shot by the walker's cannons.
The attack claimed the life of three families, including Katie Grawley.
There is some discrepancy about the timing of the massacre between the Dark Forces: Rebel Agent novel and the audio drama. The novel suggests that after the massacre, the Rebels agreed to listen to Morgan Katarn and left for Ruusan.
In the audio drama, it is suggested that Morgan somehow expected the worst and persuaded the demonstrators to leave the planet as a precaution; most of the Rebels are already far from Sulon when the Empire attacked, if not for three families who preferred to stay, and lose their lives. The same day of the event Morgan reveals to the Rebels their secret destination, and after some scepticism and complaints, he informs them about what happened that morning, to their friends who didn't follow them.