Found by Margaret Peterson Haddix

A plane mysteriously appears on a tarmac. Inside the plane, there is no pilot, no crew, no adults. However, there are 36 babies strapped into the seats and after the last baby is removed, the plane vanishes as mysteriously as it appears.

Thirteen years later, Jonah and his new friend Chip, both of them adoptees, begin to receive odd and threatening notes in the mail. When they start to investigate, with the help of Jonah's sister, Katherine, the FBI tries to discourage them with threats such as deportation. Next, strangers offering help appear and disappear mysteriously but are friends or enemies? Jonah, Chip, and Katherine scramble to find out what is happening, and, more important, to discover the true identity of the boys.

This one pulls in readers right from the start and with a cliffhanger ending, has them looking forward to the next installment.

First in a series