OAKLAND, Calif. -- Oakland Police are investigating whether the disappearance of two young girls last week was actually an attempt by a 22-year-old man now in custody to force the girls into prostitution.

The girls have since been returned to their homes and the suspect is now facing charges related to their disappearance last week.

Oakland Police said the West Oakland man arrested Tuesday night in this case faces multiple charges of lewd acts with a minor. Officers indicate more arrests may be coming in what they say appears to be a child prostitution ring.

"We're seeing a lot more of these cases where sexually exploited children are just being farmed out on these streets," Oakland Police Department spokesman Roland Holmgren.

Two girls -- ages 11 and 12 -- disappeared from Oakland's Frick Middle School last Monday. Both were originally believed to have run away from their homes.

Police located the 11 year-old on Friday -- thanks in part, they say, to a 16-year-old girl who claimed to have found her. But police say the teenager may have been aligned with the suspect to force the child into prostitution.

"We believe that perhaps one of the girls was already forced into that life and the other was being groomed," said Holmgren.

The 12-year-old girl was found Saturday. Police are now looking for a separate suspect in her disappearance.

The suspect was booked into North County Jail and is expected to face formal charges Thursday when the case is presented to the Alameda County District Attorney's Office.

Teen advocates say adult predators are increasingly targeting younger kids. Celsa Snead is executive director of the Mentoring Center in Oakland. She says poverty and a lack of family support make young girls especially vulnerable to sexual exploitation.

"The ages of the girls is getting younger and younger and the girls are more vulnerable. It's just a problem that's definitely increasing," explained Snead. "Those who are exploiting these young women are very crafty and very knowledgeable about who to pick and who to go after."

Police say they found a sawed-off shotgun and evidence that one of the missing girls had been in the suspect's home. The Alameda County District Attorney is expected to hold a press conference Thursday on the arrest.

Anyone with information regarding the case is asked call the Police Department's Youth and Family Services intake desk at (510) 238-3641.