Altar Vynn has much more to tell you than Nartholen and Walder do!
Firstly, he has discovered who your slain assassins had ties to. He can confirm, after 'aggressively persuading' an old contact of theirs, that they were employed by the Kylugo Srum fairly recently. He does not know why and has hit a block there. Nobody is willing to talk about that.
Secondly, he has sound files to send you- the unscrambled versions of the threatening messages delivered to Senator Farl's comm unit on Coruscant.
"I'm not sure if it will help," Altar says, "but I'll let you listen..."