(CNN) The last time anyone saw 97-year-old Leonie Shannon was in late December. In April, police were finally called to her Phoenix-area home for a wellness check.

Her son, who was her caretaker, told authorities she had walked away from the house in the suburb of San Tan Valley four months prior, and it wasn't the first time she had disappeared.

Police became suspicious of Daniel Shannon. They said his story changed each time he was interviewed. On Wednesday, they said, he admitted his mother died in December and he buried her in the backyard.

His motive? Police say he told them he was an inventor and needed to keep getting her Social Security and Veterans Administration checks to pay for a patent.

Pinal County Jail records show Shannon, who is in his 60s, was booked for suspicion of fraud and improper disposal of a body. It is unclear whether he has an attorney.

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