Former Utah doctor Martin MacNeill was found guilty Saturday of first-degree murder in what prosecutors called a "calculated plan" to drug and drown his wife. MacNeill, 57, had claimed the death of his wife, Michelle MacNeill, was an accident but prosecutors had charged he killed her in April 2007 to be with his mistress, CNN reported. "The defendant carried out a cold and calculated plan to murder his wife," prosecutor Chad Grunander told the jury Friday. "He relied on his knowledge and experience as a doctor and also as a lawyer to accomplish this." MacNeill's trial in Provo on charges of murder and obstruction of justice lasted 22 days. Prosecutors said the obstruction charge stemmed from the doctor's effort to hinder the police investigation of Michelle's death. They said MacNeill drugged his wife, who was 50, and then drowned her in the bathtub. A series of medical examiners who worked on the case said they could not determine precisely how Michelle MacNeill died. MacNeill's attorney, Randall Spencer told jurors the prosecution presented "cherry-picked versions of the evidence that is most consistent with their theories." Gypsy Willis, MacNeill's former mistress, testified she had an affair with him before and after his wife's death. MacNeill hired Willis shortly after his wife's death as the nanny to the four youngest of his eight children. Alexis Somers, one of MacNeill's daughters, said she was pleased by the verdict. "There was justice for my mom today," she said. "We are just so happy he cannot hurt anyone else." MacNeill faces 15 years to life in prison for murder and up to another 15 years of obstruction of justice.