Virginia State Police say Morgan Harrington may have been hitchhiking when she was last seen on the Copeley Road bridge, after leaving a Metallica concert.
However, Morgan's mother, Gil Harrington, says that would be extremely unusual behavior for her daughter.
In a telephone interview, Gil said she had never known of her daughter trying to catch a ride with a stranger.
Police say witnesses saw a woman fitting Harrington's description trying to get a ride October 17, the night Morgan disappeared.
Police say Morgan became separated from her friends after she left the concert and was denied re-entry to the John Paul Jones Arena. They say she called her friend and told them she would find a way home.