ANGEL CARROMERO, a 26-year-old youth leader in Spain’s ruling Popular Party, was the driver of a car that ran off a rural road in Cuba and crashed on July 22, killing one of the country’s leading dissidents, Oswaldo Payá, as well as another activist. Mr. Carromero denies he was at fault; a surviving passenger, a young Swedish activist, has said that “it’s wrong to accuse” him of culpability. The families of the two dissidents agree and declined to press charges against him.

Nonetheless, on Oct. 5 a Cuban court convicted the Spaniard of vehicular homicide. On Monday, he was sentenced to four years in prison. Mr. Payá‘s family was excluded from the brief trial; 42 dissidents were detained on the day it was held. The blogger Yoani Sanchez, who had driven to the town of Bayamo in order to cover it, was arrested and jailed for 30 hours.

Why did Cuban authorities respond in this way to what they describe as a one-car accident? Mr. Payá’s widow believes she knows the answer: The authorities, she charges, are trying to cover up what really happened in the crash. Family members have received accounts that the sedan Mr. Carromero was driving may have been forced off the road by another vehicle. They have called for an independent investigation with international ­involvement.

Spanish observers have their own suspicions. The regime of Raúl Castro, they say, is likely seeking to punish the ruling Spanish party for supporting the Cuban opposition. In a news conference orchestrated by Cuban authorities, Mr. Carromero and the Swedish activist said they had brought money for Mr. Payá and were helping to organize a youth movement.

Mr. Carromero’s sentence will come as no surprise to the family of Alan Gross, an American development contractor who has been a prisoner in Cuba since 2009. Mr. Gross was arrested for supplying computer equipment to Cuba’s tiny Jewish community under a U.S. aid program. Sentenced to 15 years, he has become a pawn in a gambit by the Castro regime to secure the return of five acknowledged Cuban spies who were captured and convicted of espionage in the United States.

Mr. Carromero may be in prison as a way of preventing the true story of Mr. Payá’s death from emerging, as his family believes. Or he may be a victim of a crude attempt by the Castro regime to extort concessions from the Spanish government. Spain is still attempting to obtain Mr. Carromero’s release — just as the Obama administration has tried, so far in vain, to free Mr. Gross without meeting the regime’s demands.

What’s sure is that Mr. Carromero should not be in prison because of Mr. Payá’s death. That he is offers a clear answer to those who wonder whether the Castro regime is changing for the better.