"I grabbed the computer and took it to my room, and the only other thing she could have used to send this message was my cell phone, and my cell phone is always with me," he said.
He said he suspects someone hacked her account.
But the sheriff said he doubts that. Judd said investigators don't believe the teen's Facebook account was compromised.
Mother of teen suspect arrested
Not long after Guadalupe's arrest, her mother was arrested on suspicion of child abuse and child neglect, authorities said Friday.
Judd said 30-year-old Vivian Vosburg was arrested Friday afternoon.
The sheriff emphasized that Vosburg's charges are not related to those of her daughter and the other young suspect in Rebecca's case.
Vosburg's arrest came after authorities received tips from residents that she might have been involved in beating children, Judd said. A minute-long video clip Judd played at a Friday press conference showed a woman, whom he identified as Vosburg, punching two boys with her fist while shouting profanities.
Judd said the mother first argued the beating was an accident but then said it "got out of hand" partly because she was having a bad day.
The video had been posted to the Internet and has since been taken down. It wasn't immediately clear who took the video or when and where it was taken.