>Not allowed to investigate attorneys in the department??
For now. Remember that before the Inspector General Empowerment Act of 2015 was signed by President Obama on December 16, 2016, the Inspector General's subpoena powers were very limited and the IG required permission from the heads of DOJ, FBI, etc. to investigate anything relating to those departments. This means that for Horowitz to investigate Fast & Furious, he would have needed Eric Holder's permission.