Authorities said one dog and three cats have turned up dead along the walking trail at the park in recent weeks. In the one incident, a dog and a cat were found dead, and then two cats were found dead in another incident.
In both incidents, the bodies were bound and gagged. One of the cats had been mutilated.
The area along the walking trail is surrounded by shrubs and is next to a wash. The El Monte Airport is also nearby.
"What's important is public safety," said Det. Ralph Batres of the El Monte Police Department. "It might be a prank to somebody, but to the community and for the police, we don't take it as a prank. This is serious."
Police say they have no leads on who is responsible. Anyone with information is urged to call the El Monte Police Department at (626) 580-2109.