Towards the end of Andrzej Sapkowski's The Lady of the Lake, during the parades in Novigrad as part of the peace celebrations in chapter 10, Dijkstra approaches Philippa Eilhart. He says that he has information about the assassination of king Vizimir of Redania, who was killed parallel to the incident on Thanedd Island, saying that the half-elf who did it didn't act alone.
But shortly after, when he's back in Tretogor, he gets sought out by assassins who invade the castle in search for him. Being the spy he is, he senses them in time (and successfully evades them going into hiding, as we learn later). All he says to his assistant is (translated into English by me from the official German translation):
I'm old too, and as it shows, also stupid. I said one word to one person. Only to one. And only one word. It was one word too much and one person too much.
So it seems he knows who has sent the assassins. But it is not entirely clear who it was. It seems likely that it was Philippa Eilhart, seeing how she is the only person he talked to about the assassination of Vizimir. But this in turn would mean that she herself was behind the assassination, or wouldn't it? And this could in turn mean the entire lodge of sorceresses could be involved somehow.
But it is also said that Dijkstra's assistant was later incarcerated and interrogated over the course of 6 years, so it seems there is a larger body behind this issue. So maybe someone else heard Dijkstra talk to Philippa during the parade? Or maybe he was just incarcerated by the Redanian government because of the incident on Tretogor castle from which the assassins supposedly vanished afterwards.
But my memory of the previous books is admittedly a little cloudy and I'm not entirely sure if the background behind Vizimir's assassination was maybe alluded to a little deeper earlier or if I missed anything else that clarifies the matter a little. So who sent the assassins after Dijkstra and were those the same people behind the assassination of king Vizimir? Is there anything else that I missed and that could shed a little more light on the matter or is this deliberately left ambiguous?
(If this was elaborated further in secondary material, like the games, I'd be interested to hear this, but I prefer answers concentrating on the actual book series.)