On January 11, 1959, Mari Brás founded the Pro-Independence Movement, which grouped Puerto Rican independence followers who supported the Socialist philosophy. Along with César Andreu Iglesias he founded the political newspaper "Claridad", which he directed for three decades. On November 28, 1971, at the Pro-Independence Movement's eighth general assembly, the Pro-Independence Movement officially became the Puerto Rican Socialist Party (PSP). A political organization with Marxist-Leninist and nationalist leanings influenced by the Cuban Revolution. In 1973, he spoke before the United Nations about Puerto Rico being a colony of the United States and demanded the decolonization of the island. On March 24, 1976, the eldest of Mari Brás' children, Santiago Mari Pesquera, was murdered while his father was preparing for a gubernatorial run on the Socialist Party ticket. Police investigations have hinted that Mari Pesquera was assassinated in reprisal for his father's political activism. The murder has never been officially solved. In 2009, thanks to the Freedom of Information Act documents were obtained by news stations, Primera Hora, that showed the FBI knew of a plan to assassinate Juan Marí Pesquera, and had not warned him. (They had known for two months and it was Mari Brás' son who was assassinated, in the end.)