They Never Came Back by Caroline B. Cooney (6 Book set)

     In a busy school cafeteria, a teenage girl is confronted by a classmate who questions her identity. He explains to the students who have crowded around that the girl bears an uncanny resemblance to his cousin, who was taken away by social services five years ago. Her parents abandoned her, fleeing the country after being accused of embezzling millions of dollars. The students are intrigued, but the girl shrugs off the attention as a case of mistaken identity. As the days pass, however, the boy refuses to relent and even brings his parents in to back him up. But they are not the only adults involved. An FBI agent who has been working the case these past five years believes that whoever this girl is, she can serve as bait to help the FBI capture the fugitives.