Four former Cuban political prisoners sent a letter to Democratic National Committee Chairman Tom Perez condemning presidential candidate Bernie Sanders for partially defending late dictator Fidel Castro, Breitbart reported on Tuesday.

In comments to CBS News’ “60 Minutes” on Sunday, Sanders said "It's unfair to simply say everything is bad" with the way Castro ran the country, citing as an example a literacy program he implemented.

The former political prisoners pointed out in their letter that Cuba already had high literacy rates and was the area’s wealthiest country before Castro took power and that he was responsible for the murder of thousands of dissidents, the imprisonment of many others, and the denial of freedom of expression to everyone in the nation.

“As former political prisoners who served many years in Fidel Castro’s prisons, enduring tortures, beatings, years of solitary confinement, and inhuman treatment, we are offended that a presidential candidate of your party dared to praise a tyrant who shed so much blood of innocent Cubans,” the former prisoners wrote in the letter, adding that Castro implemented a “political indoctrination program to deceive innocent peasants.”

The former prisoners demanded that Sanders apologize for his comments and warned that his “rhetoric is not only offensive to Cuban refugees and survivors of communism, but to the Democrat Party, which has long struggled to distance itself from the far left in the eyes of many Latino voters.”