In your modern-day America RPG setting, NPCs could potentially acquire materials for improvised explosive devices (IEDs) through the black market, online purchases, or even stealing from construction sites. Common materials might include fertilizers, fuel, electronic components, and easily accessible chemicals.
To detect and disrupt such plots, your police player characters might employ techniques like intelligence gathering, monitoring online activities, and collaborating with federal agencies. Additionally, they could use surveillance tools, K9 units trained to detect explosives, and implement checkpoints or random searches in areas of concern. Training in recognizing suspicious behavior and patterns could also be crucial for the police characters.
Remember to balance the realism of your game with sensitivity to real-world events and ensure your narrative promotes a positive and engaging experience for your players.