Actually, the “Official” line was that the head of the NKVD, Nikolai Yezhov, and his main advisers/subordinates, Efim Evdokimov and Mikhail Frinovsky, betrayed Stalin and carried out purges and executions behind his back. When he found out about it, he had them all sacked and ultimately executed by his new protege, Lavrenty Beria.

But seriously, what do you mean by “Official line?”. Most Historians, like Stephen Wheatcroft and Stephen Kotkin, acknowledge that the Purge was driven by a combination of Stalin’s paranoia and his desire to “shake things up” and remind everyone who was boss. Many of the Purge’s victims were Stalin’s protégés and so called “friends”. For example, Ukrainian Communist party first secretary Stanislaw Kosior and Kharkiv Bolshevik party chief Pavel Postyshev, not to mention Ukrainian NKVD leader Vsevolod Balitsky.

All three men enjoyed Stalin’s favor. Kosior was a high ranking Politburo member, and Stalin personally appointed Postyshev and Balitsky to take charge of Ukraine and increase repression during the 1932/33 famine. Postyshev would remain the highest leader in Ukraine until his downfall in early 1938, and even Kosior would have some level of influence after he fell out of favor during the Famine. They and others were purged to send the rest of the USSR party structure a message/reminder: Stalin is the boss. He made them, and he can easily unmake them, due to the absolute loyalty and NKVD had for him. Getting rid of the old guard with the purge would also make room for more fanatical up and comers. Indeed, the new leader of Ukraine would be none other than Nikita Khrushchev, formerly the Moscow party leader and the Man who would lead the USSR after Stalin’s downfall.

Stalin’s excuse was certainly Kirov’s assassination, but his motives were both his paranoia of imagined enemies, and his desire to assert control over the Bolshevik party apparatus. He did the latter by shaking things up and getting rid of followers who had outlived their usefulness (E.g. Kosior, Balitsky and Postyshev), and making room for new, more fresh up and comers (Krushchev).