The district attorney's office said on a charge evaluation sheet given to the media Friday that Jean Kasem tried to ensure that her husband was continuously provided medical care.
A potential elder abuse charge would be impossible to prove in a criminal court, the document said.
The district attorney's office said it talked over its decision with Los Angeles police, who took over the case from their Santa Monica counterparts. LAPD agreed with the decision, according to the document.
The former host of "American Top 40" was buried December 16 at Vestre Gravlund, a cemetery in Oslo, Norway, about six months after his death in a Washington state hospital. He had been suffering from Lewy body disease, the most common type of progressive dementia after Alzheimer's.
As Kasem, 82, lived out his final days, the family feuded over the best care options for him. Kasem's children from his first marriage said they were complying with their father's wishes to die comfortably and peacefully surrounded by family and friends, while Jean Kasem, his second wife, claimed they were ceding care and prematurely ending the radio legend's life.
Daughter Kerri Kasem accused her stepmother of abusing her elderly father and contributing to his death by taking him from a Santa Monica care facility in May and relocating him to Washington.
The charge evaluation sheet says police investigators found Jean Kasem wanted to get her husband away from media scrutiny.
The California legal fight continued while Casey Kasem was in Washington. Kerri Kasem was given the authority in June to have doctors end her father's infusions of water, food and medicine. Casey Kasem died just a few days later.
Kasem was already a popular disc jockey in Los Angeles when he became the host of "American Top 40" in 1970.
The syndicated show, which counted down the 40 most popular songs in the United States based on Billboard magazine's Hot 100 music chart, began on just seven radio stations but quickly became a mainstay of thousands, all around the world.
The show inspired imitators, including a television version, "America's Top 10," that Kasem hosted.