Jesse Jackson, a civil rights leader, said after a Chicago press conference on Saturday that there are unanswered questions surrounding the King of Pop's death, including about the role of the personal cardiologist who was with him.
Dr Conrad Murray reportedly was with Jackson when he stopped breathing on Thursday and performed CPR until paramedics arrived.
An ambulance crew worked on Jackson at his home for 42 minutes before rushing him to UCLA Medical Centre, where he was pronounced dead.
The cardiologist has hired a Houston-based law firm and on Saturday, an attorney there said he was cooperating.
A 911 call from Jackson's rented home reported that his personal doctor was trying to revive him without success. Police have talked to Dr Conrad Murray and have said they intend to speak with him again but have stressed he is not a criminal suspect.
Murray has yet to speak publicly since Jackson's death.
Jesse Jackson told reporters in Chicago that the "bizarre behaviour" of Murray needed explaining.
"Why did the doctor not communicate with the parents, nor with the coroner, and could not be found? It raised questions that deserve answers, and we want those answers because we are hurt."
People close to Jackson have said since his death that they were concerned about the superstar's use of painkillers.
Los Angeles County medical examiners completed an autopsy on Friday and said Jackson had taken prescription medication.
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