> The Special Prosecutions and National Security Unit is responsible for supporting counterterrorism and counterespionage investigations and prosecutions in the district. In addition, the Special Prosecutions and National Security Unit prosecutes public corruption, environmental, civil rights, child exploitation, illegal export, and immigration cases.
Regardless, if you want to know the whole story, you can read the affidavit from the link which contains literally all of the details of exactly how this played out.
tl;dr - there's really not much more to the story, the summary is exactly what happened.
Also, the affidavit makes it sound like the FBI didn't even get involved until after the company confronted him about the data they knew he kept, and him trying to use the data as leverage to get a better severance package. At that point it's really more extortion than 'hacking'.
The computer forensics part is pretty much a moot point after that.