Complex sentence: Venona also had a cover name "Charlie" that was not identified by the FBI but the 1948 "Gorsky Memo" in Soviet Archives corroborated Belfrage's covert relationship with Soviet intelligence as a member of the "Sound" and "Myrna" groups.
Split into simpler sentences: Venona also had a cover name "Charlie" that was not identified by the FBI. The 1948 "Gorsky Memo", found in Soviet Archives, identifies Belfrage as having a covert relationship with Soviet intelligence as a member of the "Sound" and "Myrna" groups.