On 17 May 1937, Largo Caballero resigned and Juan Negrín became prime minister. The NKVD-controlled secret police pursued its persecution of those who opposed the Moscow line. Antony Beevor notes that on 16 June, when the POUM was declared illegal, "the Communists turned its headquarters in Barcelona into a prison for 'Trotskyists' ... leaders were handed over to NKVD operatives and taken to a secret prison in Madrid ... Andreu Nin taken to Alcalá de Henares, where he was interrogated from 18 to 21 June ... he was then moved to a summer house outside the city which belonged to the wife of Hidalgo de Cisneros and tortured to death ... Diego Abad de Santillan remarked; 'Whether Juan Negrín won with his communist cohorts, or Franco won with his Italians and Germans, the results would be the same for us.'"