SYDNEY, Australia — The bodies were spread out across the grass — a dozen of them, legs stiff as twigs. Not a survivor could be found, it seemed, until Shai Ager saw some rustling. It was a joey just big enough to hop, trying to climb back into her dead mother’s pouch.

The next morning, in the same sports field near Cairns, Ms. Ager, an ecologist with a wallaby rescue group, found three more of the marsupials. This time they were still alive. Barely.

“They were frothing at the mouth,” she said. “I sat with them as they died.”

Who or what killed the little ones is a mystery, but with 37 wallabies now having been found dead in the same area since last Wednesday, rescuers have begun to suspect foul play. The local authorities are investigating. A toxicology report is expected in the next few days.

In the meantime, rescuers like Ms. Ager are conducting their own informal inquiry, and as is often the case in the age-old battle of humans and their housing versus habitat, the clues keep pointing to the top of the food chain.