Michael Jackson’s doctor is to be interviewed and his car has been searched as tests were ordered to see if medication caused the singer’s death.
Los Angeles Police Department assistant police chief Charlie Beck said the tests would provide “key results that will steer the direction of the investigation”.
His remark came after the city coroner’s office said a post-mortem showed no evidence of foul play in Jackson’s death.
“We are still trying to determine whether prescription drugs were involved,” Mr Beck said at a news conference.
The police chief added that his department had arranged an interview with Jackson’s private doctor, Conrad Murray, who was with the 50-year-old when he collapsed at his Bel-Air home on Thursday afternoon.
“LAPD did talk to the doctor initially but an extensive follow-up interview wasn’t carried out, so we have to do that now,” he said.
Earlier, police Dr Murray’s car was seized because detectives wanted to see if there was “any evidence of the nature of the death in that vehicle”, Mr Beck said.
It was reported by celebrity news website TMZ.com that Jackson had been receiving a daily injection of Demerol, a synthetic narcotic similar to morphine, and may have been given “too much” on Thursday.
Speaking from Los Angeles, Sky’s Robert Nisbet said: “There were rumours earlier that Dr Murray was missing, but from the news conference it would seem that he is cooperating with the police.”
Nisbet added it was interesting that the police chief had revealed the probe would extend outside the boundaries of California.
“This inquiry appears to be bigger than simply determining the sequence of events that led to Jackson’s collapse,” he said.
“It may have something to do with prescription drug use and whether he was seeing other doctors.”
Running alongside the police investigation is a report by the Los Angeles Coroner’s Office, which gave an update late on Friday evening.
Operations chief Craig Harvey told reporters the completed post-mortem showed the pop legend had not suffered any external injuries and there was no evidence of foul play.
He revealed Jackson was taking some unspecified prescription medications and said further tests had been ordered.
The extra tests would see a delay of four to six weeks before the exact cause of the performer’s death is known.
So far, it has only been revealed that Jackson went into cardiac arrest at his home and was taken to the UCLA medical centre where he was pronounced dead.
A recording of the 911 emergency call has shown Jackson was not breathing or responding to CPR when the alarm was raised.
The caller said he needed help “as soon as possible” and only the star’s personal doctor was with him when he fell ill.
During the call to the Los Angeles Fire Department the man said: “Sir, we have a gentleman here that needs help and he’s not breathing.
“He’s not breathing and we’re trying to pump him, but he’s not … he’s not breathing. He’s not conscious, sir.”
The post-mortem was reportedly performed by Dr Lakshmanan Sathyavagiswaran, who was a star witness in the O.J. Simpson trial.
Brian Oxman, a lawyer for the Jackson family, said the singer’s relatives had been concerned about his health for some time.
“Michael appeared at rehearsals a couple of times,” he said. “But his use of medications had gotten in the way.
“His injuries which he had sustained performing, where he had broken a vertebra and he had broken his leg from a fall on the stage, were getting in the way.”
The first reports of Jackson’s death emerged on the internet and sparked a huge surge in online traffic as people searched for the latest news.
TV channels around the world interrupted programmes to break the news of Jackson’s death, while many radio stations have been playing his hits back-to-back.
Singer and actress Cher said: “He was a great singer. You know, he was a genius, like Ray Charles, like Stevie Wonder. They just have this gift.”
Fans gathered near Jackson’s childhood home in Gary, Indiana, to pay their last respects to the singer who left the city years ago.
In LA, people lined the streets outside the hospital and coroner’s office while others laid flowers and candles on his star on the Hollywood Walk of Fame.
On the other side of America, fans congregated at the Apollo Theatre in Harlem, New York, where Jackson gave some of his earliest performances, to sing and dance in his memory.
Fans also made their way to the O2 Arena in London, where Jackson had been due to start his sell-out This Is It concerts in a few weeks.
Christopher Wright, who had tickets for eight of the 50 shows, said: “Everyone is in complete mourning.
“I’ve been a huge Jackson fan all my life. I was here in March when Michael announced his gigs and just wanted to come down here today.”
Rumours had spread about the state of his health when four of the comeback concerts were postponed last month.
Close friend Uri Geller suggested it may have been the stress of preparing for the shows that killed him, although this has been strenuously denied by the organisers.
See Related: MICHAEL JACKSON