WASHINGTON, March 29 ? A grand jury charged today that a former federal prosecutor in Detroit who led one of the Justice Department’s biggest terrorism investigations concealed critical evidence in the case in an effort to bolster the government’s theory that a group of local Muslim men were plotting an attack.

The prosecutor, Richard G. Convertino, and a State Department employee who served as a chief government witness were each indicted on charges of conspiracy and obstruction of justice. The grand jury charged that they had conspired to conceal evidence from the jury about photographs of an American military hospital in Jordan that was the supposed target of a terrorist plot concocted by the Detroit defendants.

Mr. Convertino, once a rising star at the Justice Department who fell out of favor with supervisors in Washington, denied that he had ever withheld evidence, and he pledged that he would be vindicated. "These charges are clearly vindictive and retaliatory, and it’s an effort to discredit and smear someone who tried to expose the government’s mismanagement of the war on terrorism," he said in a telephone interview.

The indictment of the former prosecutor and one of his star witnesses marked a striking turnaround in a case once hailed by President Bush and John Ashcroft, his first attorney general, as a major breakthrough against terrorism plotted on American soil.

After four Muslim men were arrested days after the attacks of Sept. 11, 2001, in a dilapidated Detroit apartment, federal authorities charged that they were part of a "sleeper" terrorist cell plotting attacks against Americans overseas.

Two of the men were convicted on terrorism charges after a high-profile trial in 2003, with Mr. Convertino as the lead prosecutor. But the case soon began to unravel amid allegations of concealed evidence and government misconduct. The Justice Department ultimately repudiated its own case, leading to the dismissal of all terrorism charges against the men in 2004.

"I can’t recall a case like this in recent memory where you have not only the collapse of the prosecution’s entire case, but now the prosecutor himself indicted," said Brian Levin, a professor at Cal State University at San Bernardino who has written extensively on terrorism prosecutions.

"The government has made clear it’s going to do everything it can to go after terrorism, but here you have a case where it appears that hubris might have intoxicated the prosecutor, and he might have taken one step over the line," Mr. Levin said.

Mr. Convertino, 45, who has left the Justice Department and opened his own defense practice in the Detroit area, faces a maximum of 30 years in prison and a $1 million fine if convicted. His co-defendant, Harry R. Smith III, 49, a security officer for the State Department who assisted in the prosecution, faces a maximum of 20 years in prison and a $750,000 fine.

The indictment returned by a grand jury in the eastern district of Michigan lays blame for the collapse of the case against the terrorism suspects at the feet of Mr. Convertino and Mr. Smith. It said the two men conspired "to present false evidence at trial and to conceal inconsistent and potentially damaging evidence from the defendants."

But an investigation by The New York Times published in October 2004 found that senior officials at the Justice Department knew of problems in the case almost from its inception, yet still pushed for an aggressive prosecution.

An internal Justice Department memo prepared in Washington before the 2002 indictments of the men acknowledged that the evidence was "somewhat weak," that the case relied on a single informant with "some baggage," and that there was no clear link to terrorist groups.

"We can charge this case with the hope that the case might get better," a senior counterterrorism official in Washington wrote at the time, "and the certainty that it will not get much worse."

The prosecution exposed deep rifts within the Justice Department over issues of strategy ? to the point that some Washington prosecutors assigned to work on the case were barely on speaking terms with Mr. Convertino and his Detroit prosecutors.