(CNN) -- The dark claim on the day Hugo Chavez died took many by surprise.
Someone, Venezuelan government officials said, may have deliberately infected him with cancer.
Critics dismissed the accusation -- first floated by then-Vice President Nicolas Maduro on March 5 -- as an eleventh-hour attempt to distract Venezuelans and drum up popular support as leaders prepared to announce Chavez's death.
But Maduro revived the issue this week, announcing that planning was in the works for a commission of "the world's best scientists" to investigate whether Chavez had been poisoned.
In an interview with the Telesur network hours after he registered to run for president Monday, Maduro implied that the United States could have been behind such an attack on Chavez -- an accusation that the State Department has denied.
"We have this intuition that our commander Chavez was poisoned by dark forces that wanted to be rid of him," said Maduro, who was sworn in as Venezuela's interim president on Friday.
In the 1940s and 1950s, the United States and other countries had "scientific laboratories testing how to cause cancer," Maduro said. "Seventy years have passed. These kinds of laboratories of evil and death have not advanced?"
Maduro stressed that he was not accusing the United States.
"I am just saying something that is a truth, that is known," he said.
But his recent comments have drawn sharp responses from the U.S. government.
"An assertion that the United States was somehow involved in causing President Chavez's illness is absurd, and we definitively reject it," State Department spokesman Patrick Ventrell said last week.
It isn't the first time that a Venezuelan government official has implied that a plot could be behind Chavez's cancer.
Chavez made the assertion himself in 2011, saying at a military event in Caracas that he wondered whether the United States could be infecting Latin American leaders with the illness.
Chavez cited the revelation that between 1946 and 1948, the United States had carried out human experiments in Guatemala where subjects were exposed to sexually transmitted diseases. The U.S. government apologized for those experiments in 2010.
"Would it be strange if (the United States) had developed a technology to induce cancer," he asked, "and for no one to know it?"
Maduro announced on March 5 that Chavez had died. The news came nearly two years after Chavez had revealed his cancer diagnosis in June 2011.
While government announcements of Chavez's trips to Cuba for treatment were common, officials never revealed details about his prognosis or specified what kind of cancer he had.
Maduro has remained tight-lipped, even while telling Telesur that details about Chavez's illness make officials "almost certain" that there was foul play.
"We are almost certain based on the data we have," Maduro said. "He had an illness, a cancer that will be known in time, that broke with all the typical characteristics of this illness."
Dr. Elmer Huerta, an oncologist and past president of the American Cancer Society, told CNN en Español Tuesday that assertions that injections or poisons could have caused Chavez's cancer have "absolutely no scientific substance."
"Science cannot sustain this hypothesis," Huerta said.
Any scientific investigation into Chavez's death, Huerta said, could be complicated by the fact that Chavez's body has already been embalmed so that it can be placed on display in a Caracas museum.
"If they want to investigate ... they should have already taken all the tissue samples," he said.
It's unlikely the scientific commission Maduro spoke of this week will ever be formed, according to Fernando Gerbasi, a Venezuelan analyst and former diplomat.
"This is political speculation," he told CNN en Español. "They have wanted to use to the maximum Chavez's death for political purposes."
CNN's Gabriela Frias and Mariano Castillo and CNNMexico contributed to this report.