Attorney Brian Oxman said that homicide detectives had been called in to investigate the death, and the Jackson family was looking to begin a legal battle of their own if anybody found to be responsible was not brought to justice.
The family carried out their own private second autopsy to gather evidence to start legal action, after foul play was ruled out in the first one.
"I can tell you, and this is a new development, there is a criminal investigation taking place by the Los Angeles Police Department. That is the case. They have told the family," British tabloid The Sun quoted Oxman as saying.
The King of Pop's personal doctor Conrad Murray was questioned by the Los Angeles Police Department (LAPD) on June 27.
Murray is suspected of giving the singer a shot of the potent painkiller Demerol before he collapsed in his rented Los Angeles home.
He is thought to have plunged a syringe into the star's heart in his failed attempts to save him.
Speaking about Murray, Oxman added: "I don't know what he did. But the reports I have heard are disturbing. A Demerol shot sure sounds consistent with what I knew."
A police spokesman said: "Dr Conrad Murray voluntarily contacted the Los Angeles Police Department. Detectives assigned to Robbery and Homicide Division met with him and conducted an extensive interview. Dr Murray was co-operative and provided information which will aid the investigation."