A CENTRAL premise of President Barack Obama's initiative to open relations with Cuba was that more U.S. engagement would lead to change on the island. Change is certainly needed, but recent events suggest that the unpleasant reality of Fidel Castro's dictatorship remains in place, even in the twilight of rule by his 86-year-old brother Raúl.

Twenty-one U.S. diplomats in Cuba have reported being hit with unexplained illnesses, including hearing loss, dizziness, tinnitus, visual difficulties, headaches, fatigue and cognitive, balance and sleeping difficulties. Some accounts have attributed the illnesses to strange "sonic" attacks that surfaced 10 months ago. Originally described by the State Department as an "incident," they are now being called an "attack," and Post staff writer Carol Morello reports that U.S. officials say specific Americans were targeted, that the assaults are ongoing and that they occurred in at least one case in a Havana hotel.

The State Department was prudent to decide last week to withdraw nonessential personnel and family members from Cuba, about 60 percent of the U.S. staff, as long as the danger remains and the cause is unknown. The Cuban people will be hurt by this; the United States is immediately halting the processing of visa applications by Cubans, and urging Americans not to travel to Cuba.

In a meeting Tuesday in Washington, Cuba's foreign minister, Bruno Rodríguez, told Secretary of State Rex Tillerson that Cuba wasn't at fault for the illnesses. Cuba said it had investigated but so far found no origin or cause. There has been speculation it might have been caused by a third country that came to Cuba to target the Americans. Either way, Cuba's reaction is inadequate. For decades, the Cuban state security apparatus has kept a watchful eye on everything that moves on the island, and informants lurk on every block. It begs disbelief that Cuba does not know what is going on. Unfortunately, this kind of deception and denial is all too familiar behavior. The regime took the same "don't blame us" coverup pose when the dissident Oswaldo Payá was killed in a suspicious car wreck five years ago.

Cubans who manage to escape the island are often stunned at what they find beyond it. The New York Times carried an account Friday about doctors who were sent by the government under contract to serve in Brazil and, once there, rebelled at the poor working conditions and low pay. One of them, Maireilys Álvarez Rodríguez, said how different it was to be in Brazil, "a country where you're free, where no one asks you where you're going or tells you what you have to do." These doctors are objecting to being forced to remit much of their earnings to the Cuban regime.

Cubans deserve to work freely abroad — and at home, too. They deserve to be rid of a creaky regime that now claims it has no idea who is attacking American diplomats on its soil. If Cuba sincerely wants better relations with the United States, it could start by revealing who did this, and hold them to account.