After Kouri Richins’s husband, Eric, died suddenly last year, she wrote a children’s book that she said she hoped would help her three boys process the loss, comforted by the knowledge that their father would always be with them.

On Monday, a little over two months since the book — “Are You With Me?” — was published, Ms. Richins, 33, was charged with murdering her husband by poisoning him with a lethal dose of fentanyl at their home outside Park City, Utah.

An autopsy and toxicology report determined that Mr. Richins, 39, had five times the lethal dosage of fentanyl in his system, and that the drug was illicit and not medical grade. The medical examiner also concluded that the fentanyl had been ingested orally, according to court documents.

Ms. Richins published “Are You With Me?” on March 7, just over a year after her husband’s death. A few weeks later, she appeared on KTVX-TV in Salt Lake City to promote the book, which follows the story of a boy who lost his father but is reminded of his presence.

“My kids and I wrote this book on the different emotions and grieving process that we’ve experienced in the last year, hoping it could help other kids deal with this and kind of find happiness someway or another,” Ms. Richins told KTVX. “It’s comforting for them that they’re not living this life alone. Dad is still here, but in a different way.”

A detention hearing is set for May 19. A lawyer for Ms. Richins did not immediately respond to a request for comment.

According to court documents, on the evening of March 3, 2022, Ms. Richins and Mr. Richins were celebrating the closing of a sale on a house that Ms. Richins, a real estate agent, had sold. She made her husband a Moscow mule, a cocktail, in the kitchen around 9 p.m. and brought it to their bedroom, where Mr. Richins drank it in bed.

Ms. Richins later told investigators that she went to bed and woke up around 3 a.m., because one of the boys was having a nightmare. When she returned to the couple’s bedroom, Ms. Richins told investigators, she “felt Eric and he was cold to the touch.” She called 911, according to court documents.

When Summit County sheriff’s deputies and emergency medical workers arrived, they found Mr. Richins on the floor at the foot of the bed, the court documents said. They attempted lifesaving measures but could not revive him, and he was pronounced dead.

Ms. Richins told investigators that she had left her phone plugged in next to her bed when she went to check on her son. But investigators later found that the phone had been locked and unlocked “multiple times” between the time she said she had gone to her son’s bedroom and the 911 call, according to court documents, and that she had sent and received messages during that time. The messages were later deleted, the court documents say.

In addition to murder, Ms. Richins was also charged with three counts of possession of a controlled substance, gamma-hydroxybutyric acid, with intent to distribute. Also known as GHB, it is a narcolepsy drug that is also used recreationally at dance clubs, and has been referred to as a “date-rape drug.”

A search of Ms. Richins’s phone found “several communications” with “an acquaintance” identified in court papers only as C.L. According to court documents, C.L., who has faced multiple drug charges, told investigators that Ms. Richins had reached out to him between December 2021 and February 2022, seeking help getting prescription pain medication for “an investor who had a back injury.”

Two weeks after the pain pills were delivered, Ms. Richins contacted C.L. again and said she was looking for “something stronger,” according to the court documents. She asked for “some of the Michael Jackson stuff,” specifying that she was looking for fentanyl. C.L. sold her 15 to 30 fentanyl pills for $900, the court documents say.

On Feb. 14, 2022, three days after Ms. Richins had purchased the pills, Mr. Richins became ill after a Valentine’s Day dinner at their home in Kamas, Utah. Mr. Richins later told friends that he thought his wife was trying to poison him, according to court documents.

Two weeks later, Ms. Richins contacted C.L. again, asking for another $900 worth of fentanyl pills. C.L. procured the pills from a dealer, the court documents say, and left them for Ms. Richins outside a house she was selling in Midway, Utah, south of Park City.

“Six days later,” the court documents said, “Eric was found dead of a fentanyl overdose.”