Brian Oxman, who said he is a Jackson family attorney, told CBS affiliate KCBS that the investigation is about child molestation and involves a 12-year-old boy, reports CBS News Correspondent Kelly Cobiella.
Court TV also cited unidentified sources as saying the search warrant was tied to sexual-abuse allegations brought by a 12- or 13-year-old boy. Sheriff's officials and the district attorney's office refused comment.
Jackson spokesman Stuart Backerman refused to comment on any allegations and said neither he nor Jackson knew the details of the investigation.
Sixty to 70 investigators from the Santa Barbara County sheriff's and district attorney's offices served a warrant as part of an "ongoing criminal investigation," Sgt. Chris Pappas said. No immediate arrests were made.
Backerman said Jackson and his three young children were not at the ranch at the time and have been in Las Vegas, where Jackson is making a video.
Jackson denounced media coverage of the search in a statement released by Backerman to The Associated Press.
"I've seen lawyers who don't represent me and spokespeople who do not know me speaking for me. These characters always seem to surface with dreadful allegations just as another project, an album, a video is being released," the Jackson statement said.
Detectives were expected to be gathering evidence into the night. The district attorney and sheriff planned to provide more details at Wednesday morning press conference.
In Las Vegas, a Jackson family spokesman briefly talked to reporters outside a recording studio.
"The family stands behind Michael," spokesman Steve Manning said. "He's holding up."
"It's way too early to be able to evaluate how serious a problem this is for Jackson. But, obviously, an allegation of some sort was made," says CBSNews.com Legal Analyst Andrew Cohen.
"You don't have to show very much to a judge to get a search warrant, usually it's the testimony or affidavit of a single law enforcement official who is involved in the investigation. And there are plenty of search warrants that turn out to be dead ends," he said.
Cohen added, "The document to see would be the affidavit attached to the application for a search warrant -- that document, presuming the officer involved didn't just testify in person, would have many of the details of the investigation. But don't expect it out anytime soon. It likely will be sealed until and unless there is a pending criminal case."
The 45-year-old singer who had international hits with the albums "Thriller" (1982), "Bad" (1987) and "Dangerous" (1991) saw his career begin to collapse in 1993 amid allegations he molested a boy. Jackson has maintained his innocence, and charges were never filed. He reportedly paid a multimillion-dollar settlement.
Jackson is also connected to Hollywood private eye Anthony Pellicano, who began serving federal prison time Monday for possessing illegal explosives. Pellicano is under investigation on suspicion he secretly taped conversations of celebrities and their lawyers.
Pellicano, 59, worked for Jackson as a spokesman and security consultant during the abuse investigation.
The search came on the same day Epic Records released "Number Ones," a greatest hits collection featuring Jackson's new single, "One More Chance." The song has been available at radio stations for the past few weeks, but has gotten scant airplay on pop radio stations, and has peaked Billboard's Hot R&B/Hip-Hop chart at No. 45.
Jackson's last studio album, "Invincible," sold about 2 million copies in the United States — great for most artists, especially veteran stars, but only so-so for the man who bills himself as the King of Pop.
An upcoming CBS special might boost sales of the new album. A representative for the network refused to speculate whether it would shelve the special due to Tuesday's developments.