Oltmans was based in the USA in the 1960s, where he worked as a reporter for Dutch TV broadcaster NOS and established ties to President Kennedy’s closest circle of advisors. After Kennedy was assassinated in Dallas in 1963, Oltmans interviewed the mother of accused assassin Lee Harvey Oswald, Marguerite. Further investigation led him to Oswald’s acquaintance George de Mohrenschildt. In 1977 De Mohrenschildt agreed to disclose information to Oltmans, but disappeared from their meeting place and was found dead in Florida a few weeks later.
On March 29, 1977, De Mohrenschildt was found dead due to apparent self-inflicted shotgun blast to the mouth. L. Richardson Preyer of the House Select Committee on Assassinations investigating the assassination of Kennedy stated that De Mohrenschildt was “crucial witness, based on the new information that he had”. A few days later, Oltmans told the HSCA that George de Mohrenschildt had implicated himself in the conspiracy to kill President Kennedy. And Pat S. Russell, who was De Mohrenschildt’s attorney said “I definitely feel there was a conspiracy and that definitely was the opinion of George.” Oltmans testified for three hours behind closed doors and told the committee that De Mohrenschildt told him he had discussed the assassination of Kennedy with Lee Harvey Oswald from A to Z. “De Mohrenschildt told me that Oswald acted at his (De Mohrenschildt’s) instructions and that he knew Oswald was going to kill Kennedy,” Oltmans said. Although Oltmans had given information to the Committee shortly before, De Mohrenschildt’s death had released Oltmans from his promise not to divulge certain information. Oltmans revealed that De Mohrenschildt, whom he had known for ten years, had told him that there had been a conspiracy to assassinate Kennedy and that he had played a role in the conspiracy. De Mohrenschildt said that CIA and FBI personnel were involved as well.
Oltmans played the role of De Mohrenschildt in Oliver Stone’s 1991 film about the assassination, JFK.