He and Griselio Torresola were responsible for the attempted assassination of U.S. President Harry S. Truman in Washington, D.C. on November 1, 1950.

This time the main speaker was Pedro Albizu Campos, the president of the Puerto Rican Nationalist Party.

That day Collazo was so impressed by Albizu Campos' leadership that he joined the Nationalist Party and devoted himself to it.

[3] On October 30, 1950, Torresola and Collazo learned that the Jayuya Uprising in Puerto Rico, led by the nationalist leader Blanca Canales, had failed.

Believing they had to do something for their cause, Collazo and Torresola decided to assassinate President Harry S. Truman, in order to bring world attention to the need for independence in Puerto Rico.

[1][3] On October 31, 1950, Collazo and Torresola arrived at Union Station in Washington, D.C., and registered in the Harris Hotel.

[4] During the attack, Torresola mortally wounded White House Police officer, Private Leslie Coffelt.

[2] Truman supported organizing a referendum in 1952 by which residents in Puerto Rico could vote on a proposed new constitution, which defined the island's status as an Estado Libre Associado, or Commonwealth.

In 1952, his attorney Abraham Unger petitioned for the commutation of Collazo’s life sentence for his attempted assassination of Truman.

On September 6, 1979, President Jimmy Carter commuted his sentence to time served, after Collazo had spent 29 years in prison.

President Carter also pardoned Collazo's fellow Nationalists: Irvin Flores, Rafael Cancel Miranda, and Lolita Lebrón, convicted in the 1954 United States Capitol shooting in which five members of the House of Representatives were wounded by gunfire.

Collazo's wife, Rosa, had been arrested at the time of the assassination attempt by the Federal Bureau of Investigation (FBI) on suspicion of having conspired with her husband.