() Authorities are searching for a 66-year-old woman who has been missing for nearly a month after wandering away from her tour group at the Grand Canyon.

Originally from Sault Ste. Marie, Michigan, Martha Overholser was traveling alone on a commercial bus tour. She was reported missing after failing to return on the tour.

Overholser was last seen at Mather Point on the South Rim on Dec. 10 and believed to have walked west, possibly toward Bright Angel Lodge or Hermits Rest, according to the National Park Service.

A Grand Canyon spokesperson told one local news network that Overholser’s phone was located below the rim along Hermit Road at an overlook called The Abyss. It is located about 10 miles west of Mather Point.

Overholser is white, 5’2″ tall and 110 pounds with gray hair and blue eyes. She was last seen wearing a long, off-white coat, a grey hooded sweatshirt and a navy blue bag.

Investigators are asking anyone who was at the park around that time and who may have seen Overholser to contact the National Park Service tip line at 888-653-0009 or through email at nps_isb@nps.gov.