Pundit,
The US attorney general is really the head of the department of justice. Technically a cabinet secretary. His office can initiate prosecutions for Federal crimes.
For most practical purposes though, prosecutions are done by an elected prosecutor at the county level(a subdivision of the state). The Federal Attorney General having no relevance.
Still he can initiate federal proceedings, alongside the local ones if he feels enough justification. I have seen that happen in cases where he feels an unjust outcome is likely due to the demographic and resultant jury, e.g. Trayvon Martin.