SCP ISD.

ISD is a highly secretive part of the SCP foundation. It's considered "concealed in the foundation".
Or the name is "Internal Security Department". 
I'll go over the lore. 
ISD is a concealed "foundation within the Foundation", a secret police force responsible for filtering traitors as well as operational and information security risks among the Foundation's ranks. The department is strictly hierarchical, adhering to stringently defined multi-stage investigative protocol. Potential ISD agents must have a perfect service record and a length of Foundation service of more than a year. Usually, every ISD agent keeps his "official" position in order to provide his command a first-hand account of everything happening in a given department. The very existence of ISD is usually presented as a myth. The "official" responsibility of ISD is investigation and interrogation of captured GOI agents, although that part of ISD officially belongs to Security Department for the purpose of secrecy. 
As you can see, ISD is shrouded with secrecy. 
Their duties aren't limited to internal investigations but are focused mostly to their supportive roles within the SCP Foundation. 
I got this information at this site: http://www.scp-wiki.net/list-of-foundation-s-internal-departments 
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Category:SCPS