A Utah woman charged with killing her husband with fentanyl and subsequently accused of writing a letter encouraging her brother to provide false testimony claims the letter was part of a fictional book she was writing, court documents show.

Kouri Richins is facing murder and drug charges on suspicion of killing her husband, Eric Richins, in March 2022, allegedly by giving him a drink containing a lethal dose of fentanyl. She has yet to enter a plea.

Prosecutors have said a six-page handwritten letter, which was found in Richins' cell last week, amounts to witness tampering because it allegedly contains instructions for Richins' brother to repeat "a false narrative" that would suggest her husband had gone to Mexico to buy pain pills and fentanyl, according to a motion filed last week.

But the widow claims the letter was an excerpt of a "fictional mystery book" in which she and her father go to Mexico to find drugs, according to a memorandum filed by state prosecutors on Tuesday.

Richins defended the letter in a phone call with her mother on Saturday, telling her, "When I first got in here I was telling you I was writing a book ... those letters were not a paper to you guys, they were a part of my freaking book," according to Tuesday's memorandum.

According to state attorneys, the letter instructs Richins' mother, Lisa Darden, to tell Richins' brother to falsely say that Eric Richins told him "that he got Pain Pills and fentanyl from Mexico."

"The letter claims that defense counsel, 'wants to link Eric (Richins) getting drugs and pills from Mexico' to the fentanyl that caused his death," according to prosecutors' motion last week.

The Tuesday memorandum was filed in response to a motion from Richins' defense, which argued the state's public filing of the letter violated a court-issued gag order in the case and could taint a future jury pool.

Richins' attorney, Skye Lazaro, also asserts in the defense motion that it appeared the state obtained the letter in "a potentially illegal search," saying it was found in an enveloped labeled, "Skye Lazaro (Attorney Privilege)."

Prosecutors rejected that claim, arguing instead that the letter was found inside Richins' prep book for the LSAT -- a standardized test used for law school admissions in the US -- during a legal search of her cell, according to the memorandum.

CNN has sought comment from Richin's attorneys.

Prosecutors cite 'incriminating' evidence

A judge ruled in June that Richins must remain in custody as she awaits trial, citing the "substantial evidence" against her.

Her 39-year-old husband was found dead at the foot of the couple's bed, according to authorities. An autopsy and toxicology report revealed that Eric Richins had about five times the lethal dosage of fentanyl in his system, according to a medical examiner.

Richins told investigators at the time that she had brought her husband a cocktail in the bedroom of their Kamas, Utah, home and later found him dead.

About a year later, Richins published a children's book, "Are You With Me?" about navigating grief after the loss of a loved one.

Prosecutors have cited what they say are "incriminating" internet searches made on Richins' phone, including, "What is a lethal dose of fentanyl" and "death certificate says pending, will life insurance still pay?"

Investigators have also laid out what they say is a series of illicit fentanyl purchases in the months before her husband's death, including an alleged pill delivery six days before he died, according to court documents.

Prosecutors also accuse Richins of withdrawing money from bank accounts without her husband's knowledge and trying change a life insurance policy to make herself the sole beneficiary.

Richins' lawyers have argued there was no adequate evidence proving she had purchased fentanyl, and that the state has provided no evidence that she gave her husband the fentanyl at issue.

Her attorneys also argued Kouri Richins had the right to withdraw money from their joint accounts and that "there is no evidence identifying the computer from which the login was initiated" when the life insurance policy change was attempted.