The intelligence found by the Military Department, all came from them. The specific details of the assassination, only these principals were aware of.
Especially those five big shots, excluding Duke Arondo who clearly died, whether the other four are dead or alive is completely unknown.
To want to track down the murderer, basically it's not for the Military Department to decide. Without the cooperation of these major nobles, it fundamentally won't be able to proceed.
It's not that everyone purposefully resisted the investigation, it's mainly that before the investigation had conclusively determined anything, all the families lacked even the most basic trust.
Probably at this time, they all saw each other as murderers.
Even if the trust crisis was eliminated, the Military Department would find it difficult to fully intervene. There's no choice, everybody's shady past was quite considerable.
