The AG does not have the authority to release the full report because:
1. Grand Jury material cannot be released (I think there is a limited exception and it requires the judge to do so, not the AG)
2. It was a counter-intelligence investigation. Thus there will be classified info that cannot be released.
3. The Special Counsel rules written by Congress under Janet Reno forbid it.
These restrictions on the AG are all from laws passed by Congress. Some of the current longer-term members passed these laws.
(A possible 4th would be Exec Priv, but I understand that will not asserted.)
As regards "exoneration". Investgators/prosecuters do NOT exonerate. They charge or don't charge. No where along the line in our criminal justice system is anybody ever exonerated. You are charged or not charged; you are indicted or not indicted ("true bill" or no "true bill): you are guilty or not guilty.
Fern