April 8, 2011 -- Police say they are "very concerned" about the safety of a 19-year-old woman who went to a Denver nightclub with friends and mysteriously disappeared, leaving behind her purse and cell phone but few clues.

Kenia Monge, a recent high school graduate, went to the 24K Lounge, a downtown Denver hot spot Saturday night, where a group of underage friends had been drinking. Her friends told police that she excused herself from the table to go to the restroom and never returned.

Monge was last seen several hours later at a Conoco gas station a few miles from the club at 3 a.m. Sunday, according to the Denver Police Department.

Police say the case is not a typical missing person investigation. Officials from the Denver Police Department have called in the FBI and Center for Missing and Exploited Children to help in the investigation, an indication that they do not believe Monge simply ran away.

"We get lots of missing cases," said Denver Police spokesman Lt. Matthew Murray. "No one thinks this girl ran away. We have reason to believe that what took place is not good. We are very concerned."

For the family, he added, "this is a nightmare scenario."

As the first week following Monge's disappearance comes to a close, Murray said they're treating it as a missing person case but said "there is something different" about it. He would not say what made it so strange.

Monge's stepfather, Tony Lee, told ABC affiliate KMGH that a "good Samaritan" came forward and told police that he picked up the girl and was with her at the gas station early Sunday morning where she met and left with another man.

Police would not confirm that detail, or where the "good Samaritan" had found Monge to give her a ride, but Murray told ABCNews.com they were "talking with several key witnesses. We're working every lead."

"Miracles do happen. I've seen them happen," Lee said, hoping for the best.

Monge is 4 feet, 11 inches tall, and weighs 90 pounds. She was last seen wearing a black skirt, black blouse and high-heel boots.

Friday police found a body in a suburban Denver park, but Murray said it was not Monge, nor did it have anything to do with her case.