The Only Clue — Pamela Beason

Gorillas that communicate in sign language aren't exactly commonplace, so when Dr. Grace McKenna reluctantly hosts the first Open House at her research center, half the town of Evansburg, Washington shows up to gawk. The next morning, Grace discovers that her worst nightmare has come true. All three apes -- Gumu, Neema, and baby Kanoni -- have vanished, leaving behind only one clue: a huge pool of blood.