WASHINGTON — A federal judge on Tuesday dismissed the ethics conviction of former Sen. Ted Stevens of Alaska after taking the extraordinary step of naming a special prosecutor to investigate whether the government lawyers who ran the Stevens case should themselves be prosecuted.

Judge Emmet Sullivan said that in 25 years on the bench, he had “never seen mishandling and misconduct like what I have seen” by the Justice Department prosecutors who tried the Stevens case. Sullivan’s lacerating 14-minute speech, focusing on disclosures that prosecutors had improperly withheld evidence in the case, virtually guaranteed reverberations beyond the morning’s dramatic dismissal of the verdict that helped end Stevens’ Senate career.

The judge delivered a broad warning about what he said was a “troubling tendency” he had observed among prosecutors to stretch the boundaries of ethics restrictions and conceal evidence to win cases. He named Henry Schuelke, a prominent Washington lawyer, to investigate six career prosecutors at Justice, including the chief and deputy chief of the Public Integrity Section, an elite unit charged with dealing with official corruption, to see if they should face criminal charges.

In a statement, Stevens told the court that he had long maintained an unwavering faith in the judicial system. “But what some members of the prosecution team did nearly destroyed my faith,” he said.