Complex sentence: Unfortunately, Penkovsky's activities were revealed by Jack Dunlap, a double-agent working for the KGB, who swiftly drew the conclusion that there was a mole in their ranks and set about uncovering him.
Split into simpler sentences: Unfortunately, Penkovsky's activities were revealed by Jack Dunlap, a double-agent working for the KGB. The KGB swiftly drew the conclusion that there was a mole in their ranks and set about uncovering him.