WASHINGTON • In early 1974, Do Da was top in espionage class, on the way to becoming a high-flying Central Intelligence Agency (CIA) agent: He handled himself better in the rough, carried heavier loads, and could brush off attackers.

But on his toughest-yet spy school test, he disappeared - done in by some of his own kind: ravens.

The bird was a central figure in a decade-long CIA programme to train animals as agents, helping Washington fight the Cold War.

Last Thursday, the CIA released dozens of files from its tests on cats, dogs, dolphins and on birds from pigeons to some of the smartest: ravens and crows. It studied cats as possible loose-roaming listening devices - "audio surveillance vehicles" - and put electrical implants in dogs' brains to see if they could be remotely controlled.

Neither of those programmes went very far. More effort was put into training dolphins as potential saboteurs and helping spy on the Soviet Union's development of a nuclear submarine fleet, perhaps the most potent challenge to US power in the mid-1960s.

Projects Oxygas and Chirilogy sought to see if dolphins could replace human divers and place explosives on moored or moving vessels, or swim alongside submarines to collect their acoustic signatures. Those schemes were also given up.

Another major effort was with pigeons, used for over two millennia as messengers and to take photographs during World War I.

The spy agency acquired hundreds of pigeons, testing them and cameras in areas around the US to see if they could be trained on simulated paths. Soon the target became known: the shipyards in Leningrad (now St Petersburg) where the Soviets built nuclear submarines.

After much training, the birds were brought to Washington for testing, but results were mixed. Some snapped perfect photos, but others flew out and were not seen again. One was attacked by a hawk, and came back three weeks later with no camera.

The documents do not say if the Leningrad operation was attempted. But a 1978 review the CIA released made clear that there were too many questions about the birds' reliability.

AGENCE FRANCE-PRESSE