Kilmartin joins 19 other attorneys general in a letter to U.S. Deputy Attorney General Rod Rosenstein following the dismissal of FBI Director James Comey.
Comey was leading the probe into Russian interference in the 2016 presidential election. He was fired by President Donald Trump Tuesday.
"As prosecutors committed to the rule of law, we urge you to consider the damage to our democratic system of any attempts by the administration to derail and delegitimize the investigation," read the letter.
Attorneys general from Massachusetts, Connecticut, and Vermont also signed the letter.