The eldest son of the late dictator Alfredo Stroessner has died in Paraguay, and a human rights activist said Monday the death makes it more difficult to recover a huge fortune amassed by his family.

Gustavo Stroessner, a former air force colonel, died of lung cancer Sunday, according to officials at La Costa hospital. He was 66.

Stroessner fled with his father when the dictatorship fell and spent two decades in exile in Brazil. The former dictator died in 2006 at the age of 93 in Brasilia, and his son finally returned to Asuncion last year after a judge ruled the statute of limitations on charges of illegal enrichment had expired.

The younger Stroessner's death means Paraguayans may never know what happened to a fortune that could add up to billions of dollars, human rights activist Martin Almada told The Associated Press on Monday.

"By law, when someone dies, every legal action against him also dies. So what I have left to do now is ask the attorney general of the nation to push for an investigation of his ill-gotten assets so that they can be recovered by the state," said Almada, who alleges that the family fortune was transferred to Alfredo Stroessner Dominguez, the late dictator's oldest grandson.

Stroessner Dominguez, a Colorado Party senator, was away from the capital Monday and unavailable for comment, his secretary said.

Almada is a former political prisoner who runs Paraguay's Horror Museum, where the torture tools of Stroessner's secret police are on display. He said he believes the younger Stroessner ran the family's finances and coordinated money for Paraguay's role in Plan Condor, an agreement among South America's dictatorships to eliminate leftists seeking refuge in neighboring countries.

"I'm lobbying in Brazil so that the mansion where the dictator and his son lived since 1989 is converted into a Plan Condor Museum, recalling a tragic period for democracy and human rights in Brazil, Argentina, Paraguay, Uruguay and Chile," Almada said.

Paraguay's Communist Party also released a statement Monday saying it regrets Stroessner's death because he escaped paying for "horrendous crimes."

Gustavo Stroessner's survivors also include his wife, Maria Eugenia Heikel, and sisters Graciela and Olivia.