Police in Los Angeles say the inquiry into the death of Michael Jackson could turn into a criminal case.
Police chief William Bratton told CNN that the LAPD is awaiting toxicology results.
He said investigators have spoken to doctors who treated Jackson over the years and are looking into the singer's prescription drug history.
Detectives are awaiting a coroner's report to decide how to proceed.
The Los Angeles coroner has said that an autopsy the day after the singer's death revealed no evidence of foul play and that toxicology test results could take weeks to come back.
A spokesman for the coroner's office said Jackson had taken "some prescription medication", without specifying which.
Jackson's father says he suspects foul play in Michael's death. However, Joe Jackson did not elaborate on his assertion, which was made on ABC TV News.
Jackson, 50, died on 25 June.