"We are enormously concerned," Baltimore police spokesman Anthony J. Guglielmi said Friday. "Every day that passes we get more and more freaked out."
Phylicia Barnes, a Charlotte, North Carolina, resident who was in Baltimore to visit her college-age half-sister, was last seen December 28, after saying she was going out to get something to eat and maybe a haircut, Guglielmi said.
As recently as Tuesday, police said they had no evidence that foul play was involved in her disappearance.
But now they say she has not made contact with friends or family, and hasn't turned on her cell phone, used an ATM or updated her social media sites since she disappeared.
The FBI did a profile on the girl and found no reason she would run away, Guglielmi said. She is a good student with no emotional disturbances in her life, he said.
"The fact set of this case is different than anything else we've seen," he said.
More than 100 Baltimore police, Maryland State Police troopers and FBI agents have been working around the clock to find out what happened to Barnes, Guglielmi said.
Police have questioned a dozen people believed to have had contact with her in the hours before she disappeared, and have searched the homes and cars of some of those people, he said.
They also have collected 40 hours of video from surveillance cameras at nearby businesses and apartment buildings, but neither those videos nor searches conducted in two locations have helped give investigators any leads, Guglielmi said.
On Thursday, they turned to posting billboards along I-95 in hope of gathering information about the girl's fate.
"What's going to break this thing wide open is that somebody, whether it's (in) Kansas or Canada, is going to say, wait a minute, I just saw her in a diner," Guglielmi said Friday. "That's in the best case."
Although Guglielmi said there was no evidence of illegal drug use at the apartment, the girl's mother, Janice Sallis, said she has been told the girl may have been allowed to drink alcohol and smoke marijuana at her half-sister's apartment, according to CNN and CNN affiliate WSOC.
Sallis said she had been told up to 20 men had come and gone while her daughter was there.
"I pray to God that my daughter is fine and she'll be found safe," Barnes' father, Russell Barnes, told CNN affiliate WBAL. "I hope people keep praying as high as they can and understand that this shouldn't happen to any child anywhere in this country."
Anyone with information on Barnes' disappearance can call 855-223-0033.