A teenage girl from North Carolina who was visiting her sister in Maryland went missing two weeks ago, and police say they believe she has been abducted.

Phylicia Simone Barnes, a straight-A student and track star, was visiting her half sister in Baltimore when she disappeared Dec. 28. Police said it appears Barnes left the apartment to get food around 1:30 p.m. -- and she hasn't been seen since.

A Baltimore police spokeswoman said Tuesday that authorities don't know what happened to Barnes or where she might be.

But police think she did not leave on her own accord. Barnes, whose 17th birthday is Wednesday, is known as a responsible young girl and has never tried to run away, police said.

Baltimore homicide detectives are investigating the case, and civilian volunteers helped search the grounds near Barnes' half sister's apartment over the weekend, looking for any sign of the teenager.

Barnes lives with her mother in Monroe, N.C. She was on track to graduate high school early and wanted to attend Towson University in Baltimore.

Since Dec. 28, her cell phone has gone unanswered and she hasn't used her debit card, authorities said.

Barnes' disappearance has garnered national attention and outreach efforts. Her mother, Janice Sallis, appeared on the "Today" show Saturday morning, and Nancy Grace featured a segment on the case Monday. A Facebook page, "Pray for Phylicia Barnes," has more than 14,300 supporters.

Barnes is described as a black girl who is about 5 feet 8 inches tall and 120 pounds. Authorities say she was carrying a tan purse and wearing a blue peacoat with a hood, a turquoise thermal shirt, bluejeans and white ankle boots when she went missing, according to the National Center for Missing and Exploited Children. She has a tattoo of a rose on her lower right leg.

Anyone with information about Barnes' disappearance is asked to call the Baltimore police tip line at 855-223-0033 or the NCMEC at 800-843-5678.