In an interview Friday with ABC's "Good Morning America," Joe Jackson said, "I just couldn't believe what was happening to Michael. I do believe it was foul play. I do believe that. Yes."
Jackson, however, did not say who or how his son may have been killed. He said that the autopsy commissioned by the family has yet to yield any new information.
Meanwhile, police are looking at Michael Jackson's prescription drug history and trying to talk , the Los Angeles police chief said.
William Bratton told CNN that police are waiting for the coroner's report before ruling out any possibilities in their "comprehensive" investigation into the sudden death of the 50-year-old pop star two weeks ago.
The coroner's report will determine the cause of death and hinges on time-consuming toxicology tests.
Complete coverage of Michael Jackson's life and death.
"Based on those we'll have an idea of what we're dealing with," Bratton said Thursday. "Are we dealing with homicide? Are we dealing with an accidental overdose? What are we dealing with?"
Bratton said detectives are gathering evidence, including items seized from Jackson's rented home and arranging interviews with his many physicians, but the police chief deferred to the coroner to determine the cause of death.
"The next move really is his," Bratton said. "We're not marking time waiting for his report."
The Drug Enforcement Administration and the state attorney general's office, which keeps a database of prescription drugs, are assisting investigators.
An attorney for Dr. Arnold Klein, one of Jackson's many physicians, told the Los Angeles Times that the dermatologist was subpoenaed for medical records, which he turned over to the county coroner's office.
Bratton refused to discuss details of the case.
Jackson, who died June 25, had a well-known history of using prescription medications, especially painkillers. Following his death, Cherilyn Lee, a registered nurse who had worked for Jackson, told The Associated Press she repeatedly rejected his demands for the potent anesthetic Diprivan, also known as Propofol.
Jackson had multiple doctors, friends and staff who came in and out of his life. Which people were being interviewed by police was unclear because the LAPD has said virtually nothing about the probe.
Joe Jackson said he didn't know anything about drugs his son was involved in.
"I don't even know the name of them," he said. "I do know that whatever he was taking was to make him rest because he had been working so hard."
Police towed a doctor's car from Jackson's home hours after he died and said later it could contain medication or other evidence. Coroner's officials also said Jackson was taking prescription medication but declined to elaborate.
Meanwhile, Joe Jackson told ABC that he and his wife, Katherine, should have custody of Michael's three children. "They'll grow up to be strong Jacksons," he said.
He said Michael's daughter, Paris, who spoke at the Tuesday memorial service, was taking the death hard and was crying whenever Michael's name is mentioned. He said that she might have a future in the entertainment business along with the youngest son, nicknamed Blanket, who "can really dance."