Responsa על בבא קמא 233:9

Q - Moses and his father-in-law, C, attended a banquet. A fight ensued, and S together with his brothers and brothers-in-law, drew knives and swords, and drove prominent people out of the banquet, among whom were M and C. The latter went to a Jewish court, in order to lodge a complaint. While there, they were seen whispering to X, and individual of bad repute.
Subsequently, X brought a mob of Gentiles, bent on murder and mayhem, to S's house, and X even abetted in their forced entry. As a result, S was killed.
Sometime later, S's relatives complained to the Gentile authorities whereupon they caused serious loss to M and C.
The relatives of S, claiming that M and C ordered X to bring these Gentiles to S's house, demanded that M and C be branded as murderers.
M and C, on the other hand, insist that the vilification of their name should cease, and a ban should be pronounced against anyone defaming and calumniating their name. Furthermore, they demand that the relatives of S should pay for the loss they inflicted upon M and C, expecially when the former complained to the authorities quite some time after the incident, thus voiding any claim that the relatives of S acted in the heat of anger.
A - Although M and C were seen whispering to X, when they came to a Jewish court after their eviction from the banquet, in order to lodge a complaint, there was no proof presented by valid witnesses to the effect that they induced X to have the Gentiles come and murder S. Even if they had done so, they would not be held responsible, since X should not have heeded this advice. Therefore anyone who calls M and C murderers should be rebuked.
A ban, however, should not be pronounced against any person who slanders M and C, because this ban will certainly be violated and we do not wish to "place a stumbling block before the blind".
X, who was actually responsible for inducing the Gentiles to riot and kill, should be considered a murderer, and his name should be blackened and maligned in every community that he is known.
M and C are therefore not to be accused of murder, and you may quote this decision in my name, in any community you so desire.
The Jewish courts can not hold the relatives of S liable for the losses they caused M and C by their accusations, since reparations by an informer are considered fines that can not be enforced outside of the land of Israel. Although, if such actions were to become prevalent, they would then come to be considered a form of tort called Garmi (damage which is certain to occur although done indirectly) for which the damager is held responsible, even in our day. However, thank G-d we have not heard about complaints being lodged with the authorities in cases of murder, until now, and there is therefore no fear that such complaints may become prevalent.
The claim of M and C, that the relatives of S had not acted in the rage of anger, since they had complained to the authorities quite some time after the unfortunate incident, can not be upheld - since there is no time limit in the case of murder, as to what constitutes acting in the heat of anger.
Both the relatives of S, and M and C, should leave the entire dispute in the hands of R. Shabsai b. Samuel and his court, and heed his decision in this matter.

Q. A threatens to lodge a complaint before the Duke in case his mother is successful in the suit.
A. A ban should be placed on A for even expressing such a threat; and if A dares to appeal to a Gentile court, [he shall be excommunicated, and] you shall surely excommunicate him together with his helpers, advisors, and supporters.
SOURCES: Pr. 245, 246, 247; cf. Agudah B. M. 27; ibid. B. B. 168; Hag. Maim. Shebuot 11, 3; Isserlein, Pesakim 86.