He took part in the Spanish Civil War as member of the International Brigades, serving in French or/and Polish units as a political commissar.

He was acting as agent of the Soviet intelligence NKVD and the Republican military counter-intelligence Servicio de Información Militar.

His primary task was penetration into and subversion of POUM groups in Catalonia; Narwicz was possibly related to detention of Andreu Nin.

[5] His father was involved in revolutionary work and following the Bolshevik takeover in Russia he was posted to Belarus, where he served as a commissar responsible for ensuring foodstuff delivery quotas.

[6] It is not clear who was bringing up Leon following the death of his father; according to unclear sources he led a nomadic life, perhaps as an adolescent vagrant.

This is where he got engaged in workers’ movement, taking part in rallies of the unemployed and getting in touch with the illegal Communist Party of Poland.

Detained by the police and having unclear sanctions administered against him, Narwicz left for Antwerp; he was secretary to an illegal workers organisation, grouping mostly Russian and Ukrainian migrants.

[10] It was an organization, set up in the mid-1920s, which catered to pro-Soviet Paris Russians; financed and controlled by the USSR, de facto it was a platform enabling operations of Soviet secret services, especially foreign intelligence.

British volunteers later suspected him of spying on the Independent Labour Party centre, which included copying confidential and personal documents.

[20] Before so-called May Days he supposedly got in touch with POUM leaders, including Andreú Nin;[21] he posed as a Russian communist opposed to the Stalin dictatorship.

As "León Druan" he infiltrated Sección Bolchevique-Leninista de España (SBLE), a Trotskyist organisation active in Barcelona; he reported directly to one of International Brigades leaders, Luigi Longo.

[27] Among the Barcelona Trotskyists Narwicz was identified as an undercover operative thanks to a photograph, published in the official Republican press; with his actual surname in the capture, he featured along the Communist commander and well-known Stalinist, Enrique Lister.

[28] POUM and SBLE leaders which still remained at liberty got wind that they were most likely being infiltrated by Soviet and republican special services; they also started to speculate that there was a link between Narwicz's earlier frequent visits in the headquarters and the disappearance of Nin.

[31] Tribunal d'Espionatge i Alta Traició de Catalunya, one of revolutionary jurisdiction courts set up during the war, following a brief trial condemned all of them to death penalty.

The documents in question – plus minor fragmentary pieces in recollections published by various people later – remain the only source of information on Narwicz.

So far no historian found any documents related to Narwicz in Russian, Polish,[37] German, Belgian or French archives.