Citing a lack of evidence, a San Francisco judge overturned a second-degree murder conviction Monday against Marjorie Knoller whose dogs mauled her lesbian neighbour, Diane Whipple, to death in their apartment hallway.

Superior court judge James Warren said although Knoller, 46, and her husband, Robert Noel, 60, are "the most despised couple in this city," he said the evidence did not support a murder conviction because Knoller had no way of knowing her dogs would kill someone when she left her apartment that day, according to media reports.

The judge's decision stunned the 33-year-old victim's partner, friends and relatives. Knoller's sentencing was delayed until at least July 15 so prosecutors can argue that her second-degree murder conviction should be reinstated.

Warren rejected the couple's bid for a new trial on lesser charges and allowed guilty verdicts to stand for involuntary manslaughter and owning a mischievous animal.

Noel was sentenced to the maximum four years in prison for involuntary manslaughter while Knoller who previously faced 15 years to life in prison for murder, now faces up to four years for involuntary manslaughter and having a mischievous dog that killed someone. She will be sentenced on July 15.

Knoller was with her two huge presa canario dogs when they pounced on Whipple, outside her apartment door on January 26, 2001, as she carried groceries home.

In March this year, Knoller was convicted of second-degree murder while she was also found guilty, along with her husband, of manslaughter and having a mischievous dog that killed someone.

Sharon Smith, Whipple's domestic partner, shed a tear as Warren read his ruling.

Prosecutors including San Francisco District Attorney Terrance Hallinan voiced their vehement opposition to Warren's new trial decision and said in court that they plan to argue a motion for reconsideration before Knoller's sentencing hearing. "I was surprised," he said. "I thought this went beyond manslaughter. This was a second-degree murder case."

Hallinan said: "This is a big shock for Sharon Smith that she didn't deserve on top of everything else." Warren threw out the murder conviction despite saying he did not believe much of Knoller's testimony and that Knoller and Noel who are both lawyers acted terribly in the days following the attack. He said they were cavalier about Whipple's death; the two even blamed the dead woman in pretrial interviews.

Lead prosecutor James Hammer addressed the court, "[The] grand jury chose murder for that woman over there, and 12 jurors said they were right, and you took it away. Every homicide is a tragedy. Few are as preventable as this one."