Another criminal mystery is gripping this city's zoological sleuths. Who slipped in under cover of darkness and killed the National Zoo's bald eagle, America's national symbol - and on 4 July, Independence Day no less?

Attentive readers will be well aware that the zoo is already in the hot seat over an unexplained string of losses among its star exhibits over the past three years. They include a lion, a couple of giraffes, an orang-utan, a tree kangaroo, zebras who died of malnutrition and, most recently, the poisoning of two red pandas.

Accidents and old age, said the zoo's management - these things happen. But the US Congress was not persuaded and a House Committee ordered a thorough inquiry by the National Academy of Sciences, amid calls for the replacement of the zoo's director, Lucy Spelman.

Now, however, there is the case of the bald eagle, proclaimed the national emblem by the Continental Congress in 1782, dead on America's greatest national holiday - and whose death was not accidental.

The eagle, called Captain, was found covered in mud in a corner of his enormous cage with cuts in his abdomen and his tail feathers missing. All around were signs of a fierce struggle. A half-eaten fish was lying near by. The next morning the investigators were there, going over the enclosure and making plaster casts of suspicious paw-prints, and noting a gash six inches by three in the wire mesh covering the 50ft-high cage.

But that misses the real question. Admittedly Captain was more than 20 years old, but he was able to fly and equipped with razor-sharp talons and fearsome hooked beak.

How could a bald eagle be overpowered by a creature small enough to get through the hole? And what creature would be foolish enough to try? The first suspect was a feral cat or opossum - which have been known to make unauthorised forays into animal pens in search of a free meal. But, the experts reckoned, a halfway fit eagle should have seen off an opossum, about the size of a cat, not notably aggressive and not known for its speed on the ground.

Then, they wondered, could it have been a bobcat - the North American lynx? Bobcats are ferocious fighters and could have fatally wounded the eagle.

But could a bobcat have wriggled through the hole? An additional inconvenience is that no bobcat has ever been seen in Washington, even in Rock Creek Park, the park stretching north through the city, and which is next door to the zoo.

A week later, the great whodunnit may have been solved. Strands of hair found on the wire mesh hole have been identified as belonging to a red fox. The theory is that it was a sneak attack by a predator known for its aggression, while the eagle was distracted by its lunch.

None of this diminishes the embarrassment of the hapless Ms Spelman, who is likely to come under renewed fire for poor zoo maintenance and animal care standards. Visitors, meanwhile, are urged to hurry to National Zoo, while exhibits last.