Conspiracy theorists who wonder about aliens at Roswell and Nasa faking the moon landings have a new issue to ponder: did the CIA murder Hugo Chávez?

The claim has acquired supporters since Venezuela's vice-president, Nicolas Maduro, floated it after announcing Chávez's death on Tuesday following a two-year battle with cancer.

General José Ornella, the head of the presidential guard, told AP: "I think it will be 50 years before they declassify a document [that] I think [will show] the hand of the enemy is involved."

Eva Golinger, a Caracas-based America attorney and pro-government activist, told the local paper Ultimas Noticias the US had tried to assassinate Cuba's former president Fidel Castro with radiation, among other methods, and that there was circumstantial evidence of a plot against Chavez. "We can only imagine the weapons capacity the US possesses today. They have used different biological weapons against their adversaries."

Three US groups, Partnership for Civil Justice Fund, the ANSWER Coalition and Liberation Newspaper have written to the CIA, State Department and Defence Intelligence Agency seeking documentation about any attempt to assassinate Chavez.

Maduro, who as Chavez's designated heir is favourite to win an upcoming election, said his chief had been "attacked" by the country's "historical enemies". He called for a scientific investigation. A US State Department spokesman called the claim "absurd".

Since Chavez revealed his cancer in June 2011 the exact nature of the disease, which was in the pelvic area, was treated as a state secret. Chavez claimed he was cured last year. Those who doubted that were called liars and traitors.

During his illness the president floated the idea of a CIA plot against left-wing targets, noting that leaders and former leaders in Argentina, Brazil and Uruguay had cancer. All recovered.

Chavez also said Simon Bolivar, whom history books say died of tuberculosis in 1830, was murdered by enemies in Colombia. He ordered an exhumation and tests. The conclusions did not support the theory but Chávez said he remained convinced the Liberator was murdered.