It is clear, from news reports and the report of the IRS Inspector General, that:
1.) There was a conspiracy within the IRS to delay and/or deny tax exempt status for the President's political rivals, making it difficult or impossible for them to raise money; and
2.) High level officials in the IRS were aware of this conspiracy. These officials then lied to Congress, thereby contributing to and continuing the conspiracy, until the President was safely re-elected.
What is unclear at this point, is whether other officials, in the White House and/or in the President's political organization were involved in the conspiracy.
Attorney General Holder cannot be trusted to conduct a thorough and unbiased investigation. His primary interest is to protect the President from scandal.