First adventure of my upcoming horror/espionage campaign; >

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First adventure of my upcoming horror/espionage campaign;

> party is a team hired by an unknown conspiracy to collect black magic artefacts.
> first mission is to pick up a stash of artefacts from a small office.
> rival conspiracy got there first, grabbed the stash and murdered all the office workers, fled the scene two hours before the party got there.
> every victim got their hands chopped off (for black magic purposes), some doors had their locks blasted with breaching rounds, security cameras are no good (tapes were taken/destroyed), a worker was a traitor but got killed after the stash was nicked, the safe containing the artefacts was cracked (used a cutting torch)...

I originally was thinking the party wouldn't be able to track down the rival mercs, but now I also want them to have a chance at finding them instead of just reporting back the theft to their handler.

What clues should they find on the scene? The setting's modern Earth, BTW.