Fields and his mother Samantha Bloom had moved from Kentucky to nearby OH in recent months, according to United States media reports.
Heather Heyer, 32, was killed in the attack.
He said that Fields confessed to reading and enjoying Mein Kampf, Hitler's 1925 autobiography, a book that is considered a touchstone for white supremacist and neo-Nazi groups.
Bloom's 20 year-old son, James Alex Fields Jr., was charged with second-degree murder in connection with the incident. That man, who is 20-years old, allegedly rammed his auto into a large crowd of people attending a counter-protest to the "Unite the Right" white supremacist rally.
The investigation will be conducted in Charlottesville, as well as Maumee, Ohio, Fields' hometown.
Judge Downer said the public defender's office informed him it could not represent Fields because a relative of someone in the office was injured in Saturday's protest.
In a Twitter post, the group said it had handed out the shields "to anyone in attendance who wanted them", and denied Fields was a member.
Police said he then tried to flee the scene.
Attorney General Jeff Sessions said Heyer's death "does meet the definition of domestic terrorism in our statute". The Confederate statue of Robert E Lee was in the background.
They also urged US President Donald Trump to forcefully denounce the organisations, some of which specifically cited Mr Trump's election after a campaign of racially charged rhetoric as validation of their beliefs.
Hours after the auto attack, President Donald Trump condemned bigotry in broad terms, saying that he condemned "this egregious display of hatred, bigotry and violence on many sides, on many sides".
On Saturday morning, the 20-year-old was photographed standing shoulder-to-shoulder with the far right hate group Vanguard America.