[9] An investigation led by the United States Commission on Civil Rights put the blame for the massacre squarely on the U.S.-appointed governor of Puerto Rico, Blanton Winship.

Several days before the scheduled Palm Sunday march, the Nationalists had received legal permits for a peaceful protest from José Tormos Diego, the mayor of Ponce.

According to a 1926 Puerto Rico Supreme Court ruling, government permits were not necessary for the use of plazas, parks or streets for meetings or parades.

[16] Upon learning about the march, the U.S.-appointed governor of Puerto Rico, General Blanton Winship, ordered the new Insular Police Chief, Colonel Enrique de Orbeta, to contact Mayor Tormos and have him cancel the parade permit.

[17] Without notice to the organizers, or any opportunity to appeal, or any time to arrange an alternate venue, the permits were abruptly withdrawn, just before the protest was scheduled to begin.

Rafael Molina, commanding nine policemen armed with Thompson submachine guns[13] and tear gas bombs, stood in the back.

[19] As La Borinqueña, Puerto Rico's national song, was being played, the Ponce branch of the Cadets of the Republic under the command of Tomás López de Victoria and the rest of the demonstrators began to march.

A young Nationalist cadet named Bolívar Márquez dragged himself to the wall of Santo Asilo de Damas and wrote with his blood the following message before dying:[16][23][24] ("Long live the Republic, Down with the Murderers!")

Leopold Tormes, a member of the Puerto Rico legislature, claimed to reporters that a policeman had murdered a nationalist with his bare hands.

"[26] The following day, as a result of this misinformation,[12] the New York Times and Washington Post reported that a Nationalist political revolt had claimed the lives of more than eighteen people in Puerto Rico.

After examining the photograph taken by Carlos Torres Morales of El Imparcial, which had not yet been published, he wrote a letter to Ruth Hampton, an official at the Department of the Interior.

He requested that the public prosecutor from Ponce, Rafael Pérez Marchand, "arrest more Nationalists", and that no charges be filed against the police.

A second, independent investigation ordered by the United States Commission on Civil Rights led by the ACLU's Arthur Garfield Hays, together with Puerto Rican citizens Fulgencio Piñero, Emilio Belaval, José Davila Rice, Antonio Ayuyo Valdivieso, Manuel Díaz García, and Francisco M. Zeno took place.

[13] After viewing the photograph taken by Carlos Torres Morales, Hays in his report to the American Civil Liberties Union questioned why the governor's investigation had not used the photography, which was among two that were widely published.

According to Hays, the photograph clearly showed 18 armed policeman at the corner of Aurora and Marina streets, ready to fire upon a group of innocent bystanders.

In his speech, the Congressman detailed the number of killings by the police and added, "the facts show that the affair of March 21 in Ponce was a massacre ...

[3] The year following the Ponce massacre, on 25 July 1938, Governor Winship wanted to mark the anniversary of the U.S. 1898 invasion of Puerto Rico with a military parade.

Luis Muñoz Marin and many leaders from Ponce, including attorney Pérez Marchand and some of the members of the Hays Commission were also among the founders.

The people of Puerto Rico has, [sic] for the most part, reached the conclusion, as a result of those episodes, that to be an independentista is dangerous, that to be indepedentista means persecution, damage to the person, serious financial difficulties.