"We are doing everything we can," Baltimore police spokesman Anthony Guglielmi told AOL News, noting that more than 35 detectives are working on the case, as well as two teams from the FBI.
Guglielmi added, "I just think if we could get America just to see her picture -- that is all we are asking -- maybe that will lead detectives to a break and save this young lady's life."
Phylicia lives in Monroe, N.C., but was visiting relatives in Baltimore. She was last seen around 2:30 p.m. on Dec. 28, when she left the apartment of her 27-year-old half-sister, Deena Barnes. According to relatives, Phylicia told her sister she was going shopping.
What happened to the teenager next remains a mystery, police said.
"Time is working against us," Guglielmi said. "She has been missing 10 days. ... This is not a runaway. We suspect some type of foul play. The question is, What kind of foul play?"
Investigators are preparing to examine video footage and process additional search warrants in the case, police said.
"We got some new footage [from] an apartment complex in the area, so we are reviewing that," Guglielmi said. "We are continuing interrogations, [and] we have a couple more search warrants and things to process."
Family members have described Phylicia as a straight-A student who was to graduate early from Union Academy in Monroe and planned to go to Towson University in Maryland. Her father, Russell Barnes, has said it is out of character for her to take off without notifying someone.
There has been no activity on any of Phylicia's social-networking accounts, and police say her cell phone has been turned off since the day of her disappearance.
Authorities are looking at two possible scenarios in the case, Guglielmi said. The first is that someone in the Baltimore area did "something terrible to her." The second is that she was abducted and taken elsewhere, he said.
Anyone who had contact with Phylicia in the hours leading up to her disappearance is considered a person of interest, police said. According to Guglielmi, Phylicia's half-sister's lifestyle makes the potential suspect pool large and difficult to narrow down.
"[Phylicia's] sister is college-age, and she had a lot of friends coming in and out," he said. "I would describe it probably like a dorm room setting. They party, maybe do a little drinking, [but] there is no activity there that concerned us -- no illicit drug use or sex parties or anything ridiculous. She was just a young lady with a lot of friends, [and] that makes our job difficult."
Authorities have conducted interviews with all of the individuals whom they know were present at the apartment on the day Phylicia went missing. They are now interviewing those individuals again to see if their stories are consistent, police said.
Investigators have also served a search warrant at the half-sister's apartment. Items of interest have since been sent to the FBI lab in Quantico, Va.
"We have taken out pieces of bedding and pieces of flooring, but we haven't gotten any smoking gun in terms of the case," Guglielmi said. "A couple more evidentiary things have to be processed, but it is a bit frustrating for detectives because usually you are guided by some kind of physical evidence, and in this case, so far we haven't been able to find anything."
The FBI's Baltimore branch confirmed to AOL News that it is assisting in the investigation.
"We have a unit here in Baltimore called the Crimes Against Children Unit, which is assisting the Baltimore police homicide unit in trying to locate Ms. Barnes," Special Agent Rich Wolf said.
In the past week, authorities have conducted multiple searches for Phylicia in and around the area of the apartment complex where she was last seen, but so far nothing of interest has been found. Authorities are now hoping that billboards donated by Clear Channel Communications might help bring some new leads in the case. The billboards, which display Phylicia's photo and vital information, are being put up along Interstate 95 from Baltimore to New York City, police said.
Authorities have set up a special tip line. Anyone with information on Phylicia's disappearance should call 855-223-0033. The FBI has also posted her vital information and photo on its website.