A federal judge ruled today that the FBI had no responsibility for the 1965 murder of civil rights worker Viola Liuzzo despite the presence of an FBI informer in the carload of Ku Klux Klansmen who killed her.

In a complete victory for the government, U.S. District Court Judge Charles W. Joiner rejected charges that the informer, Gary Thomas Rowe, fired the fatal shot or at least prompted the other Klansmen to do so.

"Rowe did not kill, nor did he do or say things causing others to kill," Joiner held in a 14-page opinion. "He was there to provide information and his failure to take steps to stop the planned violence by uncovering himself and aborting his mission cannot place liability on the government."

The Liuzzo family and their lawyers were stunned. The slain woman's five children had filed the $2 million lawsuit four years ago and the judge had repeatedly rebuffed government attempts to dismiss it. Joiner heard eight days of testimony at a non-jury trial earlier this year and had been weighing a ruling since April 1.

The family's chief counsel, Dean Robb, was so optimistic he had even prepared a release hailing the expected "victory for the children of Viola Liuzzo."

Mrs. Liuzzo, 39, a white housewife from Detroit, was shot to death in Alabama on the night of March 25, 1965, after a freedom march earlier that day from Selma to Montgomery. She was driving her 1963 Oldsmobile along U.S. Highway 80 when at least five shots were fired from a passing car in which Rowe and three other Klansmen were riding.

After an unsuccessful round of state prosecutions and indictments, the other Klansmen--Collie Leroy Wilkins, Eugene Thomas and William C. Eaton--were all convicted on federal charges of conspiring to violate Mrs. Liuzzo's civil rights. Eaton died of an apparent heart attack in 1966. Wilkins and Thomas served six years each, and later began pointing accusing fingers at Rowe.

A paid informer for the FBI from 1960 to 1965 and its "ace in the hole" inside the Ku Klux Klan, Rowe served a stint as the head of a Birmingham klavern's so-called "action squad" and he has publicly admitted helping to beat up black bus riders. But he always denied shooting at Mrs. Liuzzo and said he only pretended to do so. He maintained that Wilkins and Eaton shot at her car and that Thomas, who was driving, egged them on.

At the negligence trial here, both Wilkins (on videotape) and Thomas testified that Rowe had urged pursuit of Mrs. Liuzzo's car and that Rowe fired the fatal shot. A polygraph expert testified that in his opinion, the two Klansmen were telling the truth.

Judge Joiner held otherwise. He said Wilkins "is not a believable witness." As for Thomas, the judge added, his testimony "was sharply impeached" by his former wife who "testified that Thomas told her that Wilkins shot Mrs. Liuzzo at his direction."

Convinced that "Rowe did not shoot Mrs. Liuzzo" and that he actually tried to "defuse the situation without blowing his cover," the court said that the only other issue was whether FBI agents were negligent in directing Rowe to accompany the Klansmen. Describing Rowe as "a good informer, perhaps the best informer in the whole area . . . and trusted by the Klan leadership," the judge found no fault with the assignment.

Two of Mrs. Liuzzo's children, Sally, 24, and Anthony, 28, were plainly shocked when they read the decision. Sally, who said she was 6 when her mother was killed, began weeping. Anthony kept shaking his head. Later, at a news conference, he denounced Rowe as a "thug" who was "let loose by the FBI."

Robb said the opinion illustrated to him the need for amending the Federal Tort Claims Act to allow jury trials in such cases. Co-counsel Jeff Long protested that the ruling ignored the question of what restrictions the government is obliged to place on its informers. The two lawyers said an appeal will be considered.