[quirkafleeg]

> There was a post-Cold War defection of an individual (Mitrokhin) who worked for the KGB in their archives and provided a large trove of intelligence.

The article refers to that; in fact it's how this agent was identified:

    Barsky had in fact been trailed for several years
    by the FBI. His name had been discovered in files
    copied from KGB archives by Vasili Mitrokin, an 
    archivist who walked into the British embassy in 
    Riga in 1991 to offer up his secrets.