Suspected Synth

Suspected Synth at (location) is a Minutemen radiant quest in Fallout 4.

Quick walkthrough
When returning to a settlement after establishing the location for the Minutemen, the Sole Survivor will come upon a settler brandishing a weapon at another individual. The first settler is accusing the latter of being a synth. It is up to the player character to decide of what to do with the person accused.

Detailed walkthrough
Walking in from the wasteland, you can hear yelling and other signs of commotion coming from the outpost. There in the middle of the compound, a settler is aiming a weapon at another settler saying that they have been watching this one for some time and is ready to kill the synth.

The accused settler says that they are not a synth. So, it is up to the player to decide what to do with the settler.

In talking to the accused, the player will have an "Interrogate" option (medium speech check), kill the settler, let them go, or let the person accusing to decide.

Upon successful interrogation, the accused might or might not admit to being a synth. Also, if they confess to being a synth, the let him/her go option changes to where the player has to lie and say that they are not a synth. If they are not a synth, the let him/her go option remains instead. If they are not a synth and they are allowed to be killed either by the player or the accuser, a few of the other settlers will claim "Oh! I guess he/she wasn't a synth after all."

Bugs

 * It is possible to walk into the settlement to find the objective suspect dead, this seems to make the quest impossible to accomplish as the quest marker will hover above the dead suspect.
 * To fix this, you must reload a previous save and quickly go to the settlement before the Synth is killed.
 * Even after the conflict has been resolved, if the player kills the accused settler, the settlers repeat, "I guess he/she wasn't a synth after all" but become hostile nevertheless. Same can happen if the player attacks the settler along with the accuser.
 * To fix this, reload a previous save.

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