How about a massive fuel/air detonation in a city's underground sewer and storm drain system?
An FAE requires oxygen. Less of it found in a sewer system though the idea of exploding the sewer’s methane is something they could explore. As is the fact that cities have natural gas pipe infrastructure and in parts of Queens and Brooklyn here in NYC there exists jet fuel pipelines through populated neighborhoods. A very scary though limited scenario id the fact that here in NYC LIRR trains pass directly under large apartment complexes as well as the Citibank building in Long Island City. Detonate a large explosive there and 1000s could perish. Same thing with the Cross Bronx Expressway as it passes under a huge apartment complex before going on to the GWB. The fact these haven’t happened can be attributed to the skills of our anti terrorism people and the failures of our al Queda enemies. These ISIS people look like a whole new level of opposition.