The singer's June 25 death was declared a homicide due to the lethal levels of the anesthesia propofol and other drugs found in his system.
His personal physician Dr. Conrad Murray, is under investigation for administering the medication to Jackson for weeks before his death in an effort to help the allegedly insomniac entertainer sleep. Investigators are also talking to other doctors they suspect over-prescribed medications for the recording artist.
People.com quoted Joe Jackson as saying on Monday's "Today" show that until the coroner's office released its findings, he was unaware "there was foul play done."
"I didn't know all this was going on," Joe Jackson said, referring to Michael's drug use. "I'm not angry, I'm mad … Because I didn't know all this was going on."
He added that his family now "wants justice to be done."