Authorities are searching for a 66-year-old woman who went missing in Grand Canyon National Park last week after not returning to her bus tour.

       Martha Overholser, who was on a commercial tour, was last seen on Dec. 10 at Mather Point on the canyon's South Rim in Arizona, according to the National Park Service.

       Mather Point, a famous lookout, is a brief walk from the Grand Canyon Visitor Center. Overholser is believed to have walked west from the Mather Point area, possibly toward the Bright Angel Lodge or Hermits Rest, according to the National Park Service.

       She failed to return to the bus tour.

       She is described as white, 5-foot-2, with gray hair and blue eyes. She was last seen wearing a long, off-white coat and a gray hooded sweatshirt and was carrying a navy blue bag, the park service said.

       Anyone who may have seen Overholser is asked to contact officials at 888-653-0009.