The Drug Enforcement Administration was asked to help the probe by the Los Angeles Police Department, a law enforcement official in Washington told The Associated Press on condition of anonymity because of the sensitivity of the investigation.
Following Jackson's death, allegations emerged that the 50-year-old King of Pop had been consuming painkillers, sedatives and antidepressants.
The federal agency can provide resources and experience in investigating drug abuse, illicit drug manufacturers known as "pill mills" and substances local police may not be familiar with, the official said.
Also Wednesday, a plan to bury Michael Jackson at his sprawling Neverland ranch collapsed, leaving details about his funeral undecided. Another mystery was solved: His newly unveiled will says his mother should raise his children, or failing her, Diana Ross.
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The changing funeral circumstances thwarted many Jackson fans who had descended on the estate in the rolling hills near Santa Barbara with the hope of attending a public viewing.
"We're terribly disappointed," said Ida Barron, 44, who arrived with her husband Paul Barron, 56, intending to spend several days in a tent.
"We were going to listen to music and watch Michael Jackson DVDs and party all night long, not just to have fun, but in memory of Michael Jackson," Paul Barron said. "Now we're going to have to just go home."
Jackson's 7-year-old will, filed Wednesday in a Los Angeles court, gives his entire estate to a family trust and names his 79-year-old mother Katherine and his children as beneficiaries. The will also estimates the current value of his estate at more than $500 million.
Katherine Jackson was appointed their guardian, with entertainer Diana Ross, a longtime friend of Michael Jackson, named successor guardian if something happens to his mother. Ross introduced the Jackson 5 on the Ed Sullivan Show in the late 1960s and was instrumental in launching their career.
Meanwhile, Jackson family spokesman Ken Sunshine said a public memorial was in the works for Jackson but wouldn't be held at Neverland. In addition, it appeared more likely that a funeral and burial would take place in Los Angeles, a person familiar with the situation told the AP.
But the person, who is not authorized to speak for the family and requested anonymity, said nothing was planned for Neverland, at least through Friday.
The person said billionaire Thomas Barrack, who owns Neverland in a joint venture with Jackson, sought an exemption to bury the singer at the ranch. But the person says it's a complicated process and it couldn't be done for a burial this week.
"The family is aware a Neverland burial is not possible. They are expected to make decisions about whatever funeral and memorial service" will take place, the person said.
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Heavy construction equipment and workers were seen passing through the wrought-iron gates of Neverland on Tuesday. It wasn't clear what they were doing. The property is about 120 miles northwest of Los Angeles.
At once a symbol of Jackson's success and excesses, Neverland -- nestled in wine country -- became the site of a makeshift memorial after his death.
In Los Angeles, Jackson's lawyer John Branca and family friend John McClain, a music executive, were named in the will as co-executors of his estate. In a statement, they said the most important element of the will was Jackson's steadfast desire that his mother become the legal guardian for his children.
"As we work to carry out Michael's instructions to safeguard both the future of his children as well as the remarkable legacy he left us as an artist, we ask that all matters involving his estate be handled with the dignity and the respect that Michael and his family deserve," the statement said.
The will doesn't name father Joe Jackson to any position of authority in administering the estate.
The executors moved quickly to take control of all of Michael Jackson's property, going to court hours after filing the will to challenge a previous ruling that gave Katherine Jackson control of 2,000 items from Neverland.
Paul Gordon Hoffman, an attorney for the executors, told Superior Court Judge Mitchell Beckloff his clients are the proper people to take over Jackson's financial affairs. He called Katherine Jackson's speed in getting limited power over her son's property "a race to the courthouse that is frankly improper."
Judge Beckloff urged attorneys from both sides to try to reach a compromise.
"I would like the family to sit down and try to make this work so that we don't have a difficult time in court," the judge said. A hearing on the estate was set for Monday.
The will, dated July 7, 2002, gives the entire estate to the Michael Jackson Family Trust. Details of the trust will not be made public.
The documents said Jackson's estate consisted almost entirely of "non-cash, non-liquid assets, including primarily an interest in a catalog of music royalty rights which is currently being administered by Sony ATV, and the interests of various entities."
Jackson owns a 50 percent stake in the massive Sony-ATV Music Publishing Catalog, which includes music by the Beatles, Bob Dylan, Neil Diamond, Lady Gaga and the Jonas Brothers.
Jackson was recently in shaky financial health. In the most detailed account yet of the singer's tangled financial empire, documents obtained by The Associated Press show Jackson claimed to have a net worth of $236.6 million as of March 31, 2007.
Jackson, who died June 25, left behind three children: son Michael Joseph Jr., known as Prince Michael, 12; daughter Paris Michael Katherine, 11; and son Prince Michael II, 7. Debbie Rowe was the mother of the two oldest children; the youngest was born to a surrogate mother, who has never been identified.
Katherine Jackson was granted temporary guardianship Monday. A judge held off on requests to control the children's estates.
Rowe, who was married to Jackson in 1996 and filed for divorce three years later, surrendered her parental rights. An appeals court later found that was done in error, and Rowe and Jackson entered an out-of-court settlement in 2006.
Neither Rowe nor her attorneys have indicated whether she intends to seek custody of the two oldest children.
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