Successful candidates of biological weapons would want to use them despite formidable political risks, have the means to acquire them and a delivery device, and possess an organizational structure to disseminate them covertly. Terrorists are motivated to use biological weapons because they believe it would attract more attention to their cause than conventional attacks. They could be motivated to cause economic havoc or hasten their version of the apocalypse. Some aim to promote a worldwide race war and establish an Aryan state. Others want to create an aura of divine retribution. Some terrorists may want to impress their target audience with high technology or with weapons that appear more sophisticated than conventional ones. Some copy others' tactics. In 1995, Larry Wayne Harris, a white supremacist, bought vials of Yersinia pestis, the bacterium that causes bubonic plague, six weeks after the Aum Shinrikyo released sarin gas on the Tokyo subway. With the breakup of the Soviet Union, weapons of mass destruction have become easier to acquire. Microorganisms can be disseminated easily, but producing high-quality agents with the potential of inflicting mass casualties takes a great deal of sophistication. In the mid-1980's, a group known as the Covenant, the Sword, and the Arm of the Lord (CSA) acquired cyanide with the intention of poisoning the water supply. The group's leaders had not recruited technically trained personnel and chose an unworkable dissemination technique. It lacked discipline and was penetrated by the FBI. It is unlikely CSA would make such mistakes today, when groups are more aware of agents that can inflict mass casualties. The Internet links groups with information and makes terrorist acts easier to carry out. The most likely perpetrators of domestic bioterrorism are religious groups or extreme right-wing organizations. Terrorism with biological weapons is likely to remain rare, but governments cannot ignore the danger. Table, references