Several nations of the Forum tracked the projectile on a 3 dimensional radar. Tharsha's scientists retraced it and found it to be fired from the Direc Empire. We know for a fact that the Direc Empire is part of another international organization known as UNOC.
The attack was claimed by a terrorist organization. Seeing that the terrorists are in the UNOC empire, undoubtedly using their technology and were previously in HoTCR, Tharsha's officials and military leader can only conclude that UNOC is responsible for housing the terrorists, firing the railgun at orbital craft, and the biological attack on Tharsha and Raest.