The 17-year-old girl also is in custody. Authorities haven't released her name.
Naples Police Chief Mark Watkins says Harrison pummeled the girl's abdomen to try to abort the pregnancy. The six-month-old fetus survived.
Watkins says Harrison isn't the child's father.
Neither Thomas nor Watkins knew if either of the two had a lawyer.
Harrison faces one to 15 years in prison if convicted of the attempted homicide charge.
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