WASHINGTON -- Attorney General Eric Holder on Wednesday warned federal prosecutors of increased scrutiny in the wake of mistakes in the corruption case against former Alaska Sen. Ted Stevens. 

Holder told assistant U.S. attorneys for the District of Columbia that they must respond to negative perceptions of federal prosecutors by doing "the right thing." 

"Your job as assistant U.S. attorneys is not to convict people," Holder said. "Your job is not to win cases. Your job is to do justice. Your job is in every case, every decision that you make, to do the right thing. Anybody who asks you to do something other than that is to be ignored. Any policy that is at tension with that is to be questioned and brought to my attention. And I mean that." 

Holder spoke at the swearing-in ceremony for 11 assistant U.S. attorneys for the District of Columbia, an office he used to lead. His participation in the ceremony was not publicly announced by the Justice Department. An Associated Press reporter attended the open standing-room only ceremony in the federal courthouse's ceremonial courtroom. 

Holder's appearance was in the same courthouse where Stevens was freed from criminal charges a day earlier. A jury had convicted the Republican who had served 40 years in the Senate of corruption charges last fall, but Holder decided the case should be dropped because federal prosecutors withheld key evidence from Stevens' defense team. 

U.S. District Court Judge Emmet Sullivan threw out the conviction Tuesday and took the extraordinary step of ordering an investigation into whether prosecutors violated the law. Sullivan said he had never seen such misconduct and mishandling of a case during 25 years of the bench. 

He also recommended that Holder require training on evidence handling for prosecutors nationwide. 

Ken Wainstein, who served as U.S. attorney in Washington under President George W. Bush, said prosecutors already receive regular training. "The reality is that it bears constant reminding that it's more important for prosecutors to follow the rules to seek justice than to seek victories," Wainstein said. 

"And that constant reminding comes in the form of training, it comes in the form of direct supervision and it comes in the form of reminders from the department's leaders," he said. 

Steve Bunnell, a former prosecutor who taught a class at the Justice Department focusing on prosecutorial discretion, said Holder's message is a core value for federal attorneys. But he said it's an aspiration that's not always achieved in every case. 

"It's a kind of heat of battle, loss of perspective concern," said Bunnell, now a white-collar criminal attorney. "In most cases I don't think it's intentional bad faith on the part of the prosecutor. They think they are doing God's work in that moment." 

The Justice Department has been conducting its own review of the Stevens case, but Sullivan said it was taking too long and that the issues were too important to be left to an internal review. 

On Wednesday, Holder named a new head of the unit handling the review. 

Holder said the Office of Professional Responsibility will be led by Mary Patrice Brown, who leads the criminal division at the U.S. attorney's office for the District of Columbia. Holder reassigned the chief of the ethics unit, H. Marshall Jarrett, to run the executive office of U.S. attorneys. Jarrett will replace Kenneth E. Melson, who will serve as acting chief of the Bureau of Alcohol, Tobacco, Firearms and Explosives. 

Justice officials denied that the moves were related to the Stevens case. Holder did not mention the reassignments at the swearing-in ceremony, but he said he will be making changes at the department he's led for two months. 

"One of the things that we've got to really understand and get back to is that the policies that we follow have to be imbedded to and connected to our values as a nation," Holder said. "I think there's been a little distance between those two in the recent past. I don't mean to be critical, and yet I think I have to be honest with my feelings." 

He added: "There were mistakes made in the Stevens case. We'll see exactly what those mistakes were all about. 

"There are going to be a lot of articles, I suspect you're going in newspapers over the next couple of days, about how things have gone wrong in the department," Holder said. "A lot of these articles are going to be unfair. And yet we have to deal with those perceptions. We're capable of dealing with them, but the only way ultimately that we're going to restore faith in this department is through the work that you all do." 

"You are expected to do nothing more than the right thing. Anything other than that is unacceptable," he said.