Found — Margaret Peterson Haddix

You are one of the missing.

When Jonah and his best friend, Chip, start receiving threatening notes in the mail, they are plunged headfirst into a mystery. With the help of Jonah's sister, Katherine, they discover that the notes may be connected to the shady circumstances surrounding Jonah's and Chip's adoptions. When they begin to investigate, they find a vast conspiracy that reaches from the far past to the distant future -- one that will take them hurtling through time. They don't know who to trust, or which shadowy faction to believe. Can Jonah and Chip discover the secrets of their pasts before the conspiracy catches up to them?