While it is possible this was a murder by another member of the Amish community, it is relatively unlikely.
From MSNBC.com and the AP:
Girl driving buggy in Amish Ohio area fatally shot
updated 12/19/2011 7:34:32 PM ET FREDRICKSBURG, Ohio — Police say a teenage girl driving a horse and buggy in an Amish area of northeast Ohio has been shot in the head and has died. Holmes County Sheriff Timothy Zimmerly says 15-year-old Rachel Yoder died Friday, a day after her brother found her outside their home in Fredricksburg, in neighboring Wayne County.
WJW-TV reports she was Amish and had attended a Christmas party for employees of a produce shop.
Zimmerly says she dropped off a friend before being shot about 3 miles from home. He says she fell from the buggy when the horse arrived at the home.
Zimmerly tells Wooster's The Daily-Record newspaper a bullet was found in the girl's head but no bullet holes were in the buggy and no casings were nearby.
The Amish are known for careful selectivity of technology and their peaceful ideals.