Following President Donald Trump's firing of FBI director James Comey, 20 state attorneys general have added their voices to the chorus and called for an independent special prosecutor to probe alleged Russian meddling in the 2016 presidential election.

The letter (pdf), dated May 11 and addressed to U.S. Deputy Attorney General Rod Rosenstein (as Attorney General Jeff Sessions recused himself), refers to Comey's dismissal "in the middle of his investigation" of such interference "a violation of the public trust."

The letter reads:

As the chief law enforcement officers of our respective states, we view the President's firing of FBI Director James Comey in the middle of his investigation of Russian interference in the presidential election as a violation of the public trust. 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 deligitimize the investigations.

The residents of our states and the American people deserve a thorough investigation that makes clear the extent of Russian meddling, any collusion by Trump campaign officials, and any cover-up. Only the appointment of an independent special counsel, pursuant to [Justice Department regulation] 28 CFR 600.1, with full powers and resources, can begin to restore public confidence. We urge you to appoint a special counsel immediately.

President Trump's firing of James Comey was a violation of public trust. Deputy AG Rosenstein must appoint a special counsel immediately. pic.twitter.com/gwWfW0TEI0

— Maura Healey (@MassAGO) May 11, 2017

The letter was spearheaded by Massachusetts Attorney General Maura Healey, and was also signed by the attorneys general of California, Connecticut, the District of Columbia, Hawaii, Iowa, Illinois, Maine, Maryland, New Mexico, New York, North Carolina, Oregon, Pennsylvania, Rhode Island, Virginia, Vermont, Washington, Delaware, and Minnesota.

"As the top legal officers in the states, and in many cases the top law enforcement officers, it's something in which we have experience, expertise, and deep concern," Maryland Attorney General Brian Frosh said to Newsweek.

Several congressional committees and federal agencies have ongoing investigations into the matter, and Acting FBI Director Andrew McCabe said to a Senate committee Thursday that his agency's probe was still a "highly significant investigation."

Maine Attorney General Janet Mills, for her part, said, "There is growing support for an independent prosecutor to take over that investigation without the risk of meddling from any political figure (and) the attorney generals' letter is part of that growing chorus of concern."