There are numerous captives through the wasteland. It appears they have been captured by super mutants, or possibly slavers. Releasing them results in a karma boost. Care should be taken that you do not accidentally kill the captive while killing their captors. A stray missile will kill them easily, or even a frag grenade poorly thrown by a mutant.
While escaping captives may end up killed by any surviving mutants.
Captive sites can be spotted from a distance by looking for gore bags. These bags often contain ammo and caps.
Be careful around captive locations, there are often mines, rigged shotguns, or grenade bouquets around. These grenades might just accidentally kill the captive for you.
One is halfway between the Jefferson Memorial and Rivet City. It is difficult to walk between the two places without noticing a pocket of 8 or so super mutants. The mutant at the top holding a minigun stands next to the captive.
One at Hallowed Moors Cemetary. Watch out for traps.
A captive is at Mason Dixon Salvage surrounded by about 8 mutants.
Between Mason district south and Hubris comics. About 6 mutants, beware of the cars which might accidentally kill the captive.
Between Georgetown south and the mall. The captive is in the middle of the square.
There are two captives at Clifftop Shacks which is almost due north of Minefield and west of Vault 92. The super Mutants see you coming and therefore come to meet you so it's easy to dispose of them without hurting the captives.
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