Board Thread:Warhammer 40k General Discussion/@comment-24490059-20140203062149/@comment-2093048-20140520163236

Horus Heresy Book 3 has some Raven Guard get sent out to the galactic fringes, and they have a relic or helmet or something that matches what the Carcharodans have. So the Imperium's suspicions in the present day that they're Raven Guard is probably correct.