Complex sentence: In 1984, the Los Angeles office of the IRS launched a criminal investigation of L. Ron Hubbard, prompted by defectors from Scientology alleging that Hubbard had skimmed off millions of dollars.
Split into simpler sentences: In 1984, the Los Angeles office of the IRS launched a criminal investigation of L. Ron Hubbard concerning allegations of fraud. The investigation was prompted by defectors from Scientology alleging that Hubbard had skimmed off millions of dollars.