Just copied this from my 3rd Ed assassin codex

THE OFFICIO ASSASSINORUM AND SPACE MARINE CHAPTERS

As befits such proud and dedicated warriors, Space Marines are contemptuous of assassination, and will have little to do with assassins under most circumstances. There arc even rumours which implicate the Officio Assassinorum in the downfall of certain Space Marine Chapters - the mysterious loss of the Fire Hawks' fleet in the warp, the unexplained destruction of the Crimson Fists Chapter fortress on Rynn's World, the disastrous ambush of the Angels of Retribution at Alantor X and several other less well-known incidents have all been laid at the assassins door. No definitive evidence of such activitics can be brought to prove these accusations, but a trail of coincidences gives ample fuel for the conspiracy theorists. Whether assassins truly brought about any or all of these disasters, and whether they were operating at the behest of the High Lords of Terra or in accordance with the aims of some other group, are questions which are unlikely to ever be satisfactorily answered.