Googling 'fbi michael jackson' doesn't constitute research and investigation. If you can't be arsed requesting the files from the FBI, they are linked at the bottom of this article
That article you linked to @Duster12 sets out very well that it was indeed TWO sets of investigations, with 10 years BETWEEN each one. There is NOTHING to suggest it was a 10yr investigation at all. It's really something to criticize "googling" when that is plainly all you did here. The FBI didn't investigate him, they aided police forces in their investigations, again VERY different.
The following is a paragraph from the link you posted artsbeat.blogs.nytimes.com/2009/12/22/fbi-releases-files-on-michael-jackson/
The files, which were released in response to a Freedom of Information Act request made by several news media organizations, describe Mr. Jackson as “a celebrity pop star” and document how the bureau aided California law enforcement agencies when the singer was investigated for a child molestation case in 1993 [pdf] that never went to trial, and in a SECOND [my caps] such case from 2004 [pdf] in which Mr. Jackson was tried and acquitted. The files also describe ANOTHER [my caps] child molestation investigation conducted by the bureau in late 2004 [pdf] and closed when a witness, the file said, “advised the agents that he had no interest in testifying against” Mr. Jackson and “would legally fight any attempt to do so.”