Specifically Tianna Crownguard/her husband. We know now from Aftermath that Mr. Tianna Crownguard was the head of the mageseekers. We also now know that Jarvan III was going to have the mageseekers halt the arrests of mages.
We can’t say for sure that they (the crownguards) knew about that last bit, as only Xin Zhao and Jarvan IV are seen to have read the letter. What I can surmise is that someone had an idea of what Jarvan III was going to do.
Now we have also seen in Aftermath that both Tianna and Mr. Tianna (I don’t remember if they revealed his name) are advising Jarvan III on how to deal with the mage rebellion, and that can’t be good.
Edit: I’m also adding that Tianna at least seems to hold ambitions for the throne, hence recommending that Lux marry Jarvan III