LONDON -- British children’s author Helen Bailey was plied with sedatives and likely suffocated by her fiance in a financially motivated killing, prosecutors said Wednesday.

The body of 51-year-old Bailey, a successful writer of novels for young readers, was found dumped in a cesspit in the grounds of the home in Hertfordshire she shared with partner Ian Stewart in July, three months after she was reported missing. Police also found the remains of her miniature dachshund Boris near her.

Prosecutor Stuart Trimmer alleged that Stewart had fed Bailey a sleeping drug in a “long-planned, deliberate killing.” Traces of the drug Zopiclone was found to have been in Bailey’s body months before she died.

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While there were no obvious signs of physical injury, Bailey could have been killed by a “subtle” method of suffocation - or even still been alive and sedated when she was put in the septic tank, government pathologist Nat Cary said.

Stewart, 56, denies charges of murder, preventing a lawful burial, fraud and perverting the course of justice.

Bailey created the Electra Brown and Daisy Davenport series of novels for teenagers, and wrote a blog called Planet Grief after her husband of 22 years drowned during a Caribbean holiday in 2011.