So I'm doing the quest to become a BoS, and need to kill the Van Graafs.

I killed Simon their doorman so I can enter with my weapons.

However, even though I sniped him while inside a building across the road while crouching with 100 sneak and no one saw me, least of all the Van Graafs since they were all inside, somehow magically as soon as I enter the building they immediately 'know' it was me who killed Simon and start attacking me (weird, they have extra-sensory perception or what lol?)

Now I'd rather kill them quietly as is my style, perhaps pickpocketing explosives etc rather than a bloody rampage...

Is there a way to make them forget? Like perhaps waiting 3 days, or more?