Found

The Missing: Book 1 Found by Margaret Peterson Haddix

Thirteen years ago a plane suddenly appeared at the gate at the airport. It wasn't scheduled to be there and nobody knew where it had come from and where it was going. When the brand new gate agent went to investigate, she found a baby in every seat and nobody else. Today Jonah is getting strange letters in the mail telling him he is one of the missing. Then his new friend Chip starts receiving the same letters. What is going on? Who is coming for these boys? Are they really in danger? Does it have anything to do with Jonah's adoption thirteen years earlier? Jonah, Chip and Katherine, Jonah's younger sister, start to investigate but what they uncover could change the course of time forever!