The search is centered on a pond near her home. A dive team found no signs of Campbell after a search of the pond Monday, the Charlotte (N.C.) Observer reported Tuesday.
Police said they have covered an area about 7 to 8 square miles.
Authorities said she told family members Sunday afternoon she would be back in a few hours, and a family member said Campbell's bicycle, helmet and cellphone were discovered "in the area" of the pond.
"At this time we have no reason to believe there was an abduction," Mint Hill Police Lt. J. K. Rowell said Monday.