Who sent the assassins after Dijkstra?

Question

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.)}

Answer 1

Books never provide answer to that question. There are... clues (and weak ones at that), pointing to Philippa as the one who ordered it, but there are equally strong pointers that it was hierarch of Novigrad. But it could be any other ruler as well. Basically it's left to the imagination of the Reader.

However, Witcher 3 game provides answer. In one of the journal entries about Dijkstra it says in no uncertain terms that it was Philippa, after all. The spy's life story would make for a postively enthralling adventure tale. A victim of Philippa Eilhart's intrigues, he had been forced to flee Redania at breakneck speed – or have his own neck broken by assassins. For a certain time he sought refuge in far-off lands, but in the end he decided to return to the Free City of Novigrad.

EDIT Needs to be said however: Sapkowski does not consider games as "canon" - for him they are only free adaptations. Only "based on characters created by"...

EDIT 2 Hierarch of Novigrad saw it that way in Lady of the Lake:

It will be necessary, thought the hierarch, to issue a command to the kings about Dijkstra. His presence is an insult to decent people. This atheist and villain. Let him disappear without a trace. And let him be forgotten.