A group of boys killed two wallabies

If you spend much time in Dudley in the West Midlands, it is difficult to miss the zoo. It stands on a steep wooded hill, the steepest for miles, a freakish island of green among the roundabouts and redbrick and former factories. Inside the zoo, the outside world soon recedes. The chatter of the enclosures replaces the growl of traffic. The occasional keeper passes, looking deeply tanned and calm. Animals chew and stare. Paths for visitors curl and divide and disappear into the shadows.

Not even the keepers, some of who live inside the perimeter fence, claim to know everything that goes on in the zoo, especially after closing time. So when, first thing one morning in April, they found one of the zoo's half dozen wallabies with a broken leg, they were not immediately suspicious. "It was quite a severe break," says the zoo's curator, Matthew Lewis, "but it could have been thunder that spooked them." It had been raining hard in Dudley the day before. "Or it could have been a fight between the wallabies; a dominance thing."

There were also signs, though, that outside the wallaby enclosure, litter bins had been moved. Foxes were known to roam the zoo at night, even to leap into the animal pens - the fence around the wallabies had recently been raised and partly electrified to prevent this - but foxes were not strong enough to roll over the heavy bins. So the keepers set up a video camera a discreet distance from the wallabies, and trained it on their enclosure.

For a fortnight, nothing unusual was recorded. The injured wallaby was put down. Taking it away from the group for treatment was impractical, says Lewis matter-of-factly. "The others would probably have rejected it."

Then, on the morning of May 6, the keepers found the youngest wallaby, a five-month-old baby, lying in the small shed inside the enclosure where the group sheltered at night. It was soaked, muddy and dead. Outside the enclosure, the bins had been moved again. One had been rolled 50 metres uphill and placed upright beside the only section of the enclosure that was not electrified. The keepers took a look at the previous day's video tapes.