It is time for the Justice Department to stop issuing rote denials that are becoming increasingly hard to believe about the suspicious firing of eight United States attorneys. Attorney General Alberto Gonzales should appoint an impartial investigator to get to the bottom of this unfolding scandal.

Just this week, David Iglesias, one of the eight fired United States attorneys, charged that he was dismissed for resisting pressure to begin a politically charged prosecution before the 2006 election. His allegation came shortly after performance evaluations came to light that throw considerable doubt on the Justice Department’s claim that the United States attorneys were fired for poor performance.

United States attorneys, the highest federal prosecutors at the state level, must be insulated from politics. Their decisions about whether to indict can ruin lives, and change the outcome of elections. To ensure their independence, United States attorneys are almost never removed during the term of the president who appointed them.

The Bush administration ignored this tradition, and trampled on prosecutorial independence, by firing eight United States attorneys in rapid succession, including one, Carol Lam of San Diego, who had put a powerful Republican congressman in jail. Mr. Iglesias, who was the U.S. attorney in New Mexico, says two members of Congress called him last October and urged him to pursue corruption charges against a prominent Democrat before the November election. He did not. He was dismissed.