«The Lethal Gorilla» by Paul Zindel

People say New York City is a jungle. Still, P.C. Hawke is a little surprised when his best friend, Mackenzie, invites him to tag along with her police coroner mother to examine a scientist who has been mauled by jaguars at the Bronx Wildlife Conservation Park. But when the two teenage sleuths learn that the scientist actually died because he was given a lethal dose of gorilla blood after the attack, they quickly realize that the most vicious predator imaginable is loose in the park-a human killer!