Her ghastly "vision," which is how she later described it to a doctor, came to her one day in 1994, shortly after the birth of her first child, Noah. The image was fleeting but unmistakable: a knife and then someone -- she never said who -- being stabbed. Rather than tell anyone about the episode, Andrea Yates tried to forget it. But the apparition, sometimes accompanied by voices telling her to "get a knife," returned several times over the years.

Then one day last year, her attorney says, her demons took over. On the morning of June 20, Andrea's husband, Rusty, received a call from her. "You have to come home," was all she would tell him at first. And then, chillingly, "I hurt all five of the kids. I finally did it." He arrived within minutes to find police swarming around the couple's three-bedroom home in the Houston suburb of Clear Lake. An officer outside broke the awful news: Andrea had drowned their children -- Noah, 7; John, 5; Paul, 3; Luke, 2; and Mary, 7 months- -i n the bathtub. In disbelief, Rusty raced to the back door, where he saw Andrea seated impassively on the living room couch, staring blankly ahead. "I was banging on the window," recalls Rusty, who was not allowed in. " 'How could you do this?' I screamed. But she just kept looking straight ahead." She seemed to hear nothing. "I yelled again," says Rusty, " 'I don't understand. Why? Why? How could you do this?' "

Eight months later that plaintive question still hangs in the air as Yates's trial for capital murder unfolds in a Houston courtroom. And it is clear that there will be no easy answer -- if one emerges at all. The 37-year-old Yates, who pleaded not guilty by reason of insanity and could face the death penalty if convicted, "had been thinking about killing her children for two years," the state's lead prosecutor, Joseph Owmby, told the jury in his opening argument on Feb. 18. But defense attorney George Parnham, who was hired by Yates's family, depicted her as the victim of a devastating mental illness spurred in part by savage bouts of postpartum depression. Mentally unhinged after coming off Haldol, an antipsychotic drug, only two weeks earlier, Yates believed that drowning her children "was the right thing to do," Parnham told jurors. "On June 20 the inevitable happened."

If the jury of eight women and four men -- seven of whom have children -- finds Yates guilty of murder, they will then have to decide whether to condemn her to death or sentence her to life in prison with a possibility of parole in 40 years. If she is found not guilty by reason of insanity, she could still be committed to a mental hospital for life -- or until she is deemed no longer dangerous to herself or society.