Authorities are stepping up their investigation into the disappearance of a 19-year-old Colorado woman who was last seen nearly a week ago at a Denver nightclub.

The FBI Safe Streets Task Force and experts from the National Center for Missing and Exploited Children are assisting in the search for Kenia Monge, of Aurora, Colo., who was last seen by friends walking into a restroom at the 24K Lounge in downtown Denver. 

Police spokesman Matt Murray said Thursday that the disappearance of Monge isn't a standard missing-person case, but he declined to give details.

Witnesses told police that Monge left her cell phone and purse on a table before walking away to use the club's restroom.

A spokesman for the Denver Police Department told FoxNews.com that the last possible sighting of Monge was at a gas station two miles away at around 3 a.m. on April 1.

Police spokesman Sunny Jackson said that a man, whom he did not identify, claims to have offered Monge a ride home. The man told police that when he stopped at a gas station to buy cigarettes, Monge walked away with a different man, Jackson said.

"Our biggest concern is locating her," he said, adding that police are actively questioning the man who claims to have driven her in his vehicle.

"We’re not sure what he is at this point," Jackson said when asked if the man was a suspect. "It's an open investigation."

The legal drinking age in Colorado is 21. Jackson said that police are also investigating the nightclub and looking into what kind of ID, if any, Monge was carrying when she visited the club with two friends.

Johnny Coast, director of operations at 24K Lounge, confirmed to Fox 31 in an interview that Monge visited the club on March 31. But he claims Monge, whom he described as a "memorable face," was not drinking.

"Our staff has confirmed Kenia was here that evening,” Coast said. “We recognized her ... She's also been here several times before. We can also tell you she was not drinking." 

Coast also claims that Monge presented staff with an ID showing she was 21, and left the club with an unidentified man. 

"Kenia presented an ID that showed she was 21. After a few hours of dancing with the same man, she left out the back door with him. Everyone remembers it. They watched her leave with him,” he said. 

The woman's stepfather, Tony Lee, told the station that he believes Monge was drugged and taken against her will.

"She would not get into that vehicle in a million years in my opinion if she was in her right mind," Lee told the station. "There's never been any drug issues. There's never been any runaway issues, any mental health issues ... She's always been a good girl."

Monge, who moved to the U.S. from Honduras when she was 12, graduated from Cherry Creek High School last June. She is described as 4-foot-11, weighing 115 pounds. Police say she was last seen wearing a black skirt, black jacket and red high heels.

Anyone with information on Monge's whereabouts is being urged to call the Denver Police Department at (720) 913-2000. 

FoxNews.com's Cristina Corbin and The Associated Press contributed to this report