GRAND CANYON NATIONAL PARK, Ariz. (AP) — Authorities continue to search for a Michigan woman reported missing at Grand Canyon National Park.

National Park Service officials said 66-year-old Martha Anne Overholser was last seen at Mather Point on the canyon’s South Rim on Dec. 10 and later failed to return on a commercial bus tour.

Authorities said Overholser is believed to have walked west from the Mather Point area, possibly towards the Bright Angel Lodge or Hermits Rest.

Overholser is a resident of Sault Ste. Marie in Michigan, near the Canadian border.

Park officials released a missing person’s flyer about her on Thursday.