A graphic warning from Japan

In March 1995, members of a religious cult released the deadly gas sarin on the Tokyo underground system, killing 12 people and injuring more than 5,000. The gas attack was the first major terrorist chemical weapons incident and it shook the world.

The cult responsible - Aum Shinrikyo - relied heavily on universities for its recruitment, and the five main perpetrators of the attack had all graduated in science from some of Japan's most prestigious institutions.

Passengers reported seeing men with surgical masks and sunglasses leaving small boxes and bottles on three lines of the subway at 8am. The men pierced the containers with umbrellas before escaping to getaway cars.

Panic reigned during the rush hour as commuters began collapsing. Within minutes, mysterious fumes left thousands of passengers suffering convulsions and foaming or bleeding at the mouth.

Sarin was first developed by the Nazis and later used by Iraq's President Saddam Hussein against the Kurds.

It is so toxic that the absorption of 1mg through the skin or the lungs can be enough to kill. Chemical weapons experts estimate that it is 500 times more toxic than cyanide gas.

But the production of the gas is not particularly difficult - requiring chemical expertise of roughly degree level. Sarin is an organophosphorus compound similar to those used in insecticides. Its synthesis has been described in patents and textbooks.

The sect's leader, Shoko Asahara, built well-equipped laboratories in huge metal sheds on the slopes of Mount Fuji. He imported ex-Soviet technology and, over the years, thousands of tonnes of chemicals were assembled at the cult's headquarters.

Some of Asahara's previous plans to misuse science failed. Aum Shinrikyo members tried to disperse anthrax and botulism in Tokyo, but the intended mass destruction was never successful.

Asahara was sentenced to death last year. Eleven other cult members have also received death sentences, although none has been executed pending appeals.

Aum Shinrikyo continues to operate, albeit under the new name of Aleph and with a supposedly benign new remit. However, the Japanese police still monitor its activities closely and believe that it continues to pose a danger.