In 1948, as Albizu returned from prison exile and campus strikes spread, the insular legislature passed Law 53, the Ley de la Mordaza or Gag Law — a local Smith Act that made it a felony to speak, write, print, or assemble for the "overthrow" of the insular government, and in practice criminalized owning a Puerto Rican flag, singing patriotic songs, or advocating independence. It was signed by the U.S.-appointed governor and enforced for nearly a decade.
Thousands were surveilled under the notorious carpetas system and hundreds arrested; after the 1950 uprising the law became the engine of mass imprisonment, with sentences of ten years and more for speeches and membership. Repealed in 1957 and later acknowledged as flagrantly unconstitutional, the Gag Law defined an era in which the advocacy of independence was itself the crime.