Complex sentence: On July 9,2015, the estimate of the number of stolen records had increased to 21.5 million, including those of people who had undergone background checks, but not necessarily current or former government employees.
Split into simpler sentences: On July 9,2015, the estimate of the number of stolen records had increased to 21.5 million. This included records of people who had undergone background checks, but who were not necessarily current or former government employees.