Stalker: Call of Pripyat → Quests → Kopachi
Carefully take out the mercenaries and talk to Yar. He'll give you 6,000 and tell you what happened. If you want to strip the mercenaries of their equipment, you can shoot the zombies too, but then you'll have to fight them too.
On Yanov, you'll encounter a member of the Freedom faction, Uncle Yar. He asks you to escort him to the village of Kopachi. In the village, follow Uncle Yar to a house with a good sniper position on the roof. A crowd of zombies will be wandering around, fortunately, they're not aggressive. Climbing into the house's attic, we'll see the zombies engaging in a firefight with the mercenaries who have arrived.
See also:
What's in the desert? Terraria