You can choose to let Allen go free, knowing that his grand daughter relies on him, he will thank you heavily and you will earn good karma in the process.You have several ways to resolve the bounty:

Once you have the citation in your inventory, you can confront Allen on the evidence. Once the footlocker is unlocked, inside you will find 80 bottle caps, an enclave uniform, and a couple notes titled, Thank You from the Judge and Presidential Citation from the enclave president, all of which is evidence that points to Allen Rogers being Arthur Ribben (note that they also have the same initials). At the foot of Allen's bed, you will notice that there is a footlocker that is locked tight, you will need a lockpick skill of 100 in order to unlock, if not, do not worry, Allen has the key to it on him, so you will have to pickpocket him and get the key that way if your picklocking skills are unreliable. You can ask Allen if he knows anything about Arthur Ribben, but he will decline. Enter the shack and you will meet an old man named Allen Rogers, who is living with his grand daughter Sally after her father Brian died during the first battle of hoover dam and her mother abandoned her, Sally also has lost her hearing which has given Allen a reason to keep going, she makes him happy. Go to the Gibson Scrap Yard and immediately you will notice a shack on the right, next to the junkyard, called Roger's Repairs. If you collect this, you will automatically have Ribben's identity and will have a chance to confront him the moment the quest starts. Further more, if you have a Lockpick skill of 100, you can open Fisk's desk which will contain a note titled Captain Ribben which reveals the truth about Ribben's identity.

During The Head That Wears a Crown, you can find a note in Fisk's bunker titled Ridiculous by a man named Clay who encountered a woman who claims to have been held captive by Ribben and that he lives in Novac.

It also claims that Meston had the proximity searched, however the search team came to no avail.

During Loaded for Bear, in the Shadow Company Bunker you will find Meston's terminal that requires a Science skill of 100 to hack open, in it contains a dossier about Arthur Ribben, which claims they have found out that the sightings speaking of Ribben being somewhere between the 188 Trading Post and Novac are likely viable.

Indicating that Ribben resides somewhere nearby Novac. The latest report claims that Ribben was spotted doing some trading in Goodsprings and then made his way east with a caravan travelling to Novac. However, one in particular that was filed in 2280 claims that Ribben was spotted roaming the roads near the Gibson Scrap Yard.

The dossier reveals a few sighting reports, most of them are filed by nutjobs who overexagerate.Checking the Boulder City Jail Terminal for his dossier, you will find out that Ribben was born in 2210, meaning that he is at least in his early 70's and that the image of him on the wanted poster is outdated.Over the course of the mod, you will find various clues to Ribben's whereabouts and occupation as you go through bounties. You can ask Joaquin for some information regarding Ribben, however the only information he knows is outdated. Read the poster at the Boulder City Jail, this is a loaded quest as you have to try and figure out Ribben's location yourself. To collect the bounty, submit Arthur Ribben or proof of his death to Captain Larry Scull at the Boulder City Jail. He is to be considered armed and dangerous, and should only be pursued by licensed professionals. He is wanted for genocide, conspiracy, and other crimes.

The NCR is offering a bounty of 2,000 caps for the capture of Arthur Ribben.

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