This paper presents an online, search-augmented large-scale language model (LLM) framework to overcome the limitations of existing scenario generation methods in simulation-based testing, which is essential for autonomous vehicle (AV) verification. Using an LLM-based behavior analyzer, we infer the most dangerous intents of background vehicles and generate feasible adversarial trajectories via additional LLM agents. As new intents are encountered, we automatically extend the behavior library of intent-planner pairs via dynamic memory and search storage to mitigate forgetting and accelerate adaptation. Evaluation results using the Waymo Open Motion Dataset demonstrate that it outperforms existing methods, reducing the average minimum collision time from 1.62 s to 1.08 s and reducing the collision rate by 75%.