Feds have no jurisdiction over this. Barr cannot do anything unless a Federal law was violated.
How was James Fields convicted, then? The feds have countless possible charges to bring in cases like this, from conspiracy/RICO charges to ‘civil rights intimidation’ charges. Thus far, they are choosing not to act against violent leftists. This is a policy choice, not a matter of law.
What if these anarchists are receiving funding, across state lines?