A supporter waves a Cuban flag in front of the country's newly reopened embassy in Washington. (Mark Wilson/Getty Images)

PRESIDENT OBAMA’S opening to Cuba is based on the hope that, after more than a half-century of hostility from the United States, a surge of commerce, information and travel will somehow erode the rigid authoritarian state built by Fidel Castro and now presided over by his brother, Raúl. The assumption is questionable: The opening, including Monday’s reestablishment of embassies, could well enhance rather than undermine the regime. The United States has diplomatic relations with many authoritarian governments that flout human rights, including China, Saudi Arabia and Russia. What will matter with Cuba is not the raising of flags in Washington and Havana but how the United States applies its influence.

Negotiations that led to this point included “a pretty robust conversation” about the abysmal human rights situation in Cuba, a senior administration official told reporters last week. Cuba has released some political prisoners. But frequent reports from the island make it plain that routine harassment continues of dissidents and those who speak out. Short-term detentions and beatings are common, especially when the courageous Ladies in White, a group of wives and female relatives of jailed dissidents, take to the streets after Sunday Mass.

The opening of embassies upgraded the status of U.S. officials in Havana, who will now be full-fledged diplomats, with freedom to move around Cuba. Perhaps they should skip a diplomatic reception or two and use this newfound access to examine one of the most unsettling chapters in the history of the Castro dictatorship.

Three years ago Wednesday, on July 22, 2012, the Cuban dissident Oswaldo Payá was riding in a car with an associate, Harold Cepero, and two foreign visitors, from Spain and Sweden. The Spaniard, Ángel Carromero, a leader of the youth wing of Spain’s ruling party, was driving the blue rental car down a remote country road on the way to visit activists in Mr. Payá’s Christian Liberation Movement. The car crashed, killing Mr. Payá and Mr.Cepero.

Mr. Carromero was accused of vehicular homicide and jailed after a show trial in Havana where he was pressured into saying that the car had crashed because of his reckless driving. Mr. Carromero was later released to Spain, and has since declared, in an interview with us and in a book, that the car was forced off the road by another vehicle bearing Cuban government license plates. His statements suggest that Mr. Castro’s goons caused the crash that killed Mr. Payá and Mr. Cepero.

Unfortunately, no government or international institution has carried out a credible investigation of Mr. Payá’s death. On Wednesday the Human Rights Foundation of New York will publish a report highlighting many legal and factual questions that linger. Then there is the political legacy: More than a decade ago, Mr. Payá’s Varela Project received thousands of signatures for a petition calling for a referendum on legal reforms that would liberalize Cuba’s political system. Now that he’s gone, others are carrying on the fight inside Cuba, and suffering for it. They, rather than the Castro regime, should be the focus of U.S. diplomacy.