Story highlights Doctor: There is "no scientific substance" to claims that Chavez was deliberately infected

President Maduro says he will invite "the world's best scientists" to study Chavez's illness

The interim leader says he suspects Chavez "was poisoned by dark forces"

The State Department denies accusations of U.S. involvement, calling them "absurd"

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?"

Photos: Photos: Political career of Hugo Chavez Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Army Lt. Col. Hugo Chavez, who led a 1992 attempted coup, speaks to reporters on March 26, 1994, after he was freed from jail. Chavez was freed after charges were dropped against him for leading the first of two attempted coups against the government of former President Carlos Andres Perez, who was later removed from office. Hide Caption 1 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Venezuelan president-elect Chavez visits Bogota, Colombia, on December 18, 1998. On December 6, Chavez had been elected the youngest president in Venezuela history. Hide Caption 2 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – President Chavez greets supporters with his then-wife, Marisabel Rodriguez de Chavez, beside him as he arrives to preside over a parade in his honor on February 4, 1999, in Caracas. Chavez was sworn in as president on February 2. Hide Caption 3 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez inspects military maneuvers of the national Air Force on March 17, 2001, in Catilletes near the border with Colombia. In June 2000, Chavez was re-elected to the presidency for a six-year term, under the new constitution created by his government in 1999. Hide Caption 4 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – People try to take shelter from gunshots fired near Altamira Square in Caracas on August 16, 2004. At least three people were wounded by gunshots after Chavez supporters fired on opposition demonstrators, police said. A vote to recall Chavez as president failed on August 15. Hide Caption 5 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez, left, stands in front of supporters with Fidel Castro of Cuba, center, and Evo Morales of Bolivia, right, during a rally at the Plaza de la Revolucion in Havana, Cuba, on April 29, 2006. Hide Caption 6 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez meets with Pope Benedict XVI at his private library on May 11, 2006, in Vatican City. Hide Caption 7 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez embraces Iranian President Mahmoud Ahmadinejad, right, in Tehran, Iran, on July 1, 2007. The two presidents have enjoyed a close relationship and Chavez has referred to Ahmadinejad as his "ideological brother." Hide Caption 8 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez speaks during a rally in Caracas on November 18, 2008. Chavez pushed to change term limits in Venezuela through a referendum that passed on February 15, 2009, clearing the way for him to run for a third six-year term. Hide Caption 9 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez, right, gives a copy of the book, "The Open Veins of Latin America" by Eduardo Galeano to President Barack Obama during a multilateral meeting at the Summit of the Americas in Port of Spain, Trinidad, on April 18, 2009. Hide Caption 10 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez, right, greets Russian Prime Minister Vladimir Putin during his visit to the presidential palace in Caracas on April 2, 2010. Hide Caption 11 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez salutes to the audience after passing a law in Caracas on November 12, 2011. Chavez has undergone several rounds of cancer treatment in Cuba, beginning in 2011. Hide Caption 12 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez participates in a ceremony at the Miraflores presidential palace in Caracas on January 27, 2012. Hide Caption 13 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez, left, jokes with American actor Sean Penn, right, during his visit to Miraflores presidential palace in Caracas on February 16, 2012. Penn thanked Chavez for the support given by the Venezuelan government to his nongovernmental organization, which benefits victims of the 2010 Haiti earthquake. Hide Caption 14 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez acknowledges supporters on the streets of Caracas while on his way to the airport to travel to Cuba for ongoing cancer treatment on February 24, 2012. Hide Caption 15 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez gestures to the crowd during his closing campaign rally in Caracas on October 4, 2012. The leftist leader won a fourth term on October 7, extending his presidency to 2019. Hide Caption 16 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – A handout picture released by the Venezuelan presidential press office on Friday, February 15, 2013, shows Chavez surrounded by his daughters and holding the February 14 edition of the official Cuban newspaper Granma at a hospital in Havana, Cuba. Hide Caption 17 of 18 Photos: Photos: Political career of Hugo Chavez Political career of Hugo Chavez – Chavez supporters gather in Caracas' Bolivar Square to mourn Chavez's death on March 5, 2013. Hide Caption 18 of 18

JUST WATCHED What is Hugo Chavez's economic legacy? Replay More Videos ... MUST WATCH What is Hugo Chavez's economic legacy? 04:37

JUST WATCHED iReporters remember Hugo Chavez Replay More Videos ... MUST WATCH iReporters remember Hugo Chavez 01:15

JUST WATCHED GPS: Venezuela after Chavez Replay More Videos ... MUST WATCH GPS: Venezuela after Chavez 01:52

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."