Shortly after beginning, the player must speak to NPC Iliud who says "A merchant's son was kidnapped... The perpetrators of this terrible act remain unmasked, but evidence at the scene suggests..."

In the context of the incident being described, and the RDM quest line altogether, it should probably say "remain masked" because the perpetrators had not been identified or caught. Though, "remain masked" doesn't sound entirely natural, so maybe "have yet to be unmasked" or simply "remain unknown".