After Goa's annexation by the
Indian Army in 1961, Monteiro was recruited by the
Polícia Internacional e de Defesa do Estado (PIDE), the Portuguese Secret Service. Initially, he participated in a campaign to organise an anti-Indian resistance among Goans code named Plan Gralha. PIDE agents used the Emissona Nacional, radio station which continued to broadcast after 1961, to broadcast stories of Goan non-cooperation with Indian authorities. Plans were made to sabotage port facilities at Mormugao and Bombay. In 1962, a bomb was planted at Vasco Municipal School in March 1962, and another at a different location in October. Casimiro was implicated in bomb blasts on June 20, 1964. The Portuguese government, in an official statement to the UN, later claimed that the attacks were acts of revolt against Indian rule. In 1965, he assassinated General Humberto Delgado in Spain. General Delgado was the Opposition Leader against the Salazar government. Monteiro, who had shot Delgado and strangled his secretary, was found guilty by the Spanish courts and was sentenced to 19 years
in absentia. Monteiro crossed into Tanzania, and using a parcel bomb assassinated
Eduardo Mondlane, the founding leader of FRELIMO. ==Later life==