A small group of experts [were] enlisted by A.I. companies to vet their products for catastrophic risks. Some have shared with The Times more than a dozen chatbot conversations revealing that even publicly available models can do more than disseminate dangerous information. The virtual assistants have described in lucid, bullet-pointed detail how to buy raw genetic material, turn it into deadly weapons and deploy them in public spaces, the transcripts show. The U.S. government has long planned for powerful adversaries unleashing deadly bacteria, viruses or toxins in the American population. Since 1970, there have been a few dozen, fairly small biological attacks around the world, such as the anthrax-laced letters that killed five Americans in 2001. Despite perennial warnings, a major catastrophe has not happened and remains unlikely, most experts say. But even if the probability is low, an effective biological weapon could have an enormous impact. Dozens of experts told The Times that A.I. is one of several recent technological advances that have meaningfully increased that risk by expanding the pool of people who could cause harm. Still, A.I. users would need some real-world expertise to follow a bot's instructions. Some research, including a study backed by A.I. companies, has found that while chatbots can help novices learn certain lab skills, the technology isn't particularly helpful for carrying out the range of complex tasks needed to make a virus from scratch.