As a PIDE agent (the secret police of the Portuguese dictatorship), Monteiro carried out the high-profile assassinations of Humberto Delgado, who was a political opposition leader to Salazar's "Estado Novo"; and Eduardo Mondlane, leader of FRELIMO; in addition to this he also brutally tortured an uncertain number of Goans who advocated for the union of Goa with India, something which did happen on mid-1954 with the Indian annexation of Dadra and Nagar Haveli and on December 1961 with the Indian annexation of Goa.