"He did not commit suicide. That was just not consistent with his character," she said. "I think he was murdered."
Her father died after plunging from his yacht off the Canaries - named Lady Ghislaine in her honour. According to pathologists, his body was found floating, which was inconsistent with drowning.
Ms Maxwell said: "He did not commit suicide because he didn't drown. There is no evidence of suicide. It was not unhelpful to certain parties that the suicide theory was put about, because it meant his insurance policy would be in dispute."
Ms Maxwell, who now lives in New York, also said she was still grieving for the loss of her father.
"I am definitely prone to tears," she said.