A romance novelist who wrote about “How to Murder Your Husband” was convicted in her husband’s killing on Wednesday following a contentious trial in which prosecutors leaned on a “puzzle” of circumstantial evidence to portray the author as a duplicitous spouse who spent months quietly plotting the perfect crime.

Nancy Brophy, 71, stood quietly, a pandemic mask covering her nose and mouth, as the verdict was handed down, seven weeks after the trial began in Portland, Ore.

Prosecutors had built their case with evidence showing that Ms. Brophy had acquired gun pieces in the months before the killing of her husband, Daniel Brophy, including one extra component that prosecutors said could ensure that the bullets used in the shooting would not be traced back to her gun. Prosecutors contended that Ms. Brophy shot her husband in his workplace, where there would be no cameras or witnesses, then moved to collect on lucrative life insurance policies in the days that followed.

“She had the plan in place,” Shawn Overstreet, a deputy district attorney, said in closing arguments this week. “She had the opportunity to carry out this murder. She was the only person who had the motive.”

The second-degree murder charge for which Ms. Brophy was convicted carries a punishment of life in prison. She is scheduled to be sentenced June 13