Brian Murtagh was a junior Army legal officer in 1971, fresh from George Washington University law school, when he was asked to review one of the most publicized murders of the decade.

He not only became involved in the case, but also came to be the lead prosecutor in the murder trial of Capt. Jeffrey MacDonald, the Green Beret physician accused of fatally beating and stabbing his wife and two daughters a year earlier at Fort Bragg, N.C.

Today the bespectacled lawyer is still in law enforcement back in the District of Columbia, where he deals with cases that tend to have a much lower profile.

"I'm working where I want to work," said Murtagh, 39, an assistant U.S. attorney, "doing what I want to do."

MacDonald contended the murders had been carried out by drug-crazed hippies.

Asked simply to initial an Army investigative report on the case, which the service had decided to drop, Murtagh instead took the time to read it. The rest was documented in the best-selling book "Fatal Vision" by Joe McGinniss and later in an NBC television movie of the same name.

For the remainder of his Army hitch, and for years afterward at the Justice Department, Murtagh -- collaborating with MacDonald's father-in-law, Alfred Kassab -- struggled to persuade authorities to reopen the case and prosecute MacDonald for the deaths. MacDonald, convicted in 1979, is serving three life sentences in a federal prison in Texas.

Last month, the 4th U.S. Circuit Court of Appeals in Richmond denied a request by MacDonald for a new trial.

"No one at Justice believed, I think, that we might win," Murtagh said of his long fight to revive the case. "It was written off as a dog. A dog that had to be tried, but a dog."

Murtagh found himself unable to drop the investigation. For the 25-year-old lawyer who had never been in a courtroom, it was "literally the first case" he was involved in. But more importantly, he said, "The horror of the thing is something I remembered. I don't have sleepless nights over it, but I remember when I did. The crime scene pictures -- they're bad."

Murtagh also was disturbed by the way evidence had been lost or misplaced during the investigation's early stages. "The government had such a bad track record of institutional failings," he said. "I wasn't going to add my name to the list of those who'd dropped the ball."

Since his conviction, MacDonald, who once referred to Murtagh as "that viper," has waged a steady publicity campaign to clear his name. A group of friends and supporters in California recently launched a newsletter devoted to winning the former doctor's release.

That effort, said the prosecutor, is "totally inappropriate and totally useless."