UPDATE: The Delaware County DA’s Office says Riley Harkins has been found safe.

MEDIA, Pa. (CBS) — Authorities have launched a major search for a missing teenager in Delaware County. Police in Media believe 15-year-old Riley Harkins, who has been missing for 11 days, is in danger. Police sources believe Riley is possibly being held against her will.

From tips and leads, the search for the missing girl has intensified and is now urgent. Fifteen Delaware County detectives are now on the case and working around the clock.

The last significant tip of Riley’s whereabouts was in Bristol at the SEPTA train station.

Riley is now considered to be an endangered person.

Her mother and other family members have been wallpapering areas where they believe the Penn Crest student has been staying.

Police say Riley left Media with a 15-year-old friend 11 days ago.

The investigation shows Riley and her friend took a train into the city and eventually met up with at least two other people who were older.

Riley’s friend surfaced a couple of days after leaving Media, but there has been no sign of Riley.

The friend told police that the two had been taken to a home in the Kingsessing section of Southwest Philadelphia and police sources believe the missing teen is possibly being held against her will.

“We don’t even know if she’s alive or dead,” Riley’s mother, Kristen Evans Harkins, said. “We just know there are different leads and people running in different directions, but we want Riley to know we’re looking for her, won’t stop. We want her to be safe. We want to know that she’s safe.”

“Any time we get info a kid has left the home for a significant period of time, we start getting information, meeting with family and friends,” Delaware County District Attorney Katayoun Copeland said. “What’s most disconcerting is when Riley ran away with a friend and her friend returned and Riley didn’t. That’s when it became most disconcerting from our vantage point, a safety and security vantage point.”

The case was initially characterized as a runaway juvenile, but investigators are alarmed Riley has not returned home, leading them to believe she is being held against her will.

If you know anything or see anything, police urge you to call 911.