"We have this intuition that our commander Chavez was poisoned by dark forces that wanted to be rid of him," Nicolas Maduro, sworn in as interim president Friday, told Venezuela's pan-Latin American TV network teleSUR.
"The world's best scientists" will be invited to join a government panel to probe the accusation, Maduro said.
Maduro, 50, implied Washington could have been behind such an attack -- an accusation the State Department has flatly denied.
He said in the 1940s and 1950s, Washington and other powers had "scientific laboratories testing how to cause cancer."
"Seventy years have passed," Maduro said. "These kinds of laboratories of evil and death have not advanced?"
Maduro stressed he was not accusing Washington of causing Chavez's cancer. "I am just saying something that is a truth, that is known," he said.
Maduro also suggested Washington might have been behind Chavez's cancer March 5, shortly before he announced Chavez's death, after he announced the expulsion of two U.S. Embassy officials.
Maduro -- Chavez's handpicked successor, running as his party's candidate in an April 14 election triggered by Chavez's death -- said there would someday be "scientific proof" Chavez was infected by outsiders.
"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 March 5.
Maduro told the teleSUR Tuesday the way Chavez's illness developed had all but convinced Venezuelan officials foul play was behind it.
"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."
Chavez himself suggested in 2011 he might have been poisoned, saying at a military event in Caracas, the capital, he wondered if Washington could be infecting Latin American leaders with cancer since several had developed it.
Chavez died at age 58, 21 months after revealing his cancer diagnosis in June 2011.
Dr. Elmer Huerta, an oncologist and past president of the American Cancer Society, told CNN en Espanol Tuesday Maduro's assertions had "absolutely no scientific substance."
"Science cannot sustain this hypothesis," Huerta said.