NOT SINCE Herbert Philbrick, who led three lives for the FBI, has an undercover agent for the bureau received as much attention as Gary Thomas Rowe Jr. Mr. Rowe infiltrated the Ku Klux Klan in 1960 as a paid FBI spy. In 1965, he was one of four Klansmen cruising around Lowndes County, Ala., looking for trouble at the time of the Selma civil rights marches. Someone in that car fired a bullet that killed Viola Liuzzo, a white mother of five who had volunteered to come from Detroit to help organizers of the march.

Because Mr. Rowe was in the car and was able to notify the FBI within hours of the shooting, arrests were soon made. But murder prosecutions were unsuccessful, resulting first in a mistrial and then an acquittal of the Klansman who Mr. Rowe said fired the shot. In both trials, the jury was all-white; at the second trial, a majority of the jurors were members of the local White Citizens' Council. When murder prosecutions failed, the three Klansmen were tried and convicted on the federal charge of civil rights violations, and each was sentenced to 10 years in prison. Informer Rowe was something of a minor hero at the time.

Then the questions began. If Mr. Rowe had been so close to the Klan that he knew of its planned activities, did he take part in or try to stop the Birmingham church bombing in 1963 that killed four little girls? Why did he participate in the beating of freedom riders in Birmingham in 1961? And had he really once killed a black man as he claimed in a 1978 interview? Had the FBI done everything possible to supervise and restrain this unpredictable informer?

Some of these same questions have arisen again in a $2 million lawsuit against the government filed by the family of Viola Liuzzo in a federal court in Michigan. One can take with a grain of salt the testimony at that trial that Mr. Rowe himself fired the fatal shot, since it comes from those who were originally tried in the case and who went to jail on Mr. Rowe's testimony. But the hard question of the government's liability when its agent is present at a crime is the one that the Michigan judge must decide.

Informers will always be needed by federal law enforcement officials, especially in criminal cases where mob activity or terrorism is involved. And the kind of people who, almost by definition, are best able to do this work are those who can convince others that they, too, are criminal and violent. The number of undercover agents placed in domestic organizations has been greatly reduced since Attorney General Edward Levi issued guidelines for these FBI investigations in 1976. There is a risk, of course, that without a wide net of informers in such groups, prosecutions will be difficult. But of equal concern is the risk that inadequately supervised or even criminally inclined informers will themselves undermine instead of aid the government's efforts to enforce the law.