Once this murder has been carried out, another letter is dispatched requesting the secret murder ( i.e. Usually the serial killings must only go on so long as it takes the Brotherhood to discover the identity of the killer, at which point they dispatch a courier with a letter requesting the murder of a specific person. When a spate of murders is committed in cold blood and without an arrest, the Brotherhood instigates an investigation to recruit the killer.
Additionally, the Dark Brotherhood has been known to recruit through proxy. Secondly, when an Argonian from Black Marsh is born under the sign of the Shadow, they are trained by an order known as the 'Shadowscales,' and pending the end of their training, transferred to the Dark Brotherhood. The next time the murderer sleeps in a location deemed to be secure, a Speaker will approach the murderer and ask them to join the Dark Brotherhood, and if the offer is accepted, the murderer will then be given a specific target to kill. When a person commits murder, it is likely that the Night Mother has been watching from the shadows.