As STRATFOR predicted in mid-May, the threat of a terrorist attack inside the United States is intensifying . U.S. counterterrorism sources are greatly concerned about an attack this summer, as demonstrated by the May 26 government warnings, and a global effort to disrupt attack plans with public alerts domestically and a concerted push for arrests and extraditions abroad. Passenger rail and subway systems remain vulnerable to Madrid-style attacks. In addition to the detonation of conventional explosive devices on public transport, U.S. counterterrorism officials say the possibility for a chemical attack is increasing. Chemicals could be the weapon of choice among militant operatives. This week, STRATFOR intelligence sources have expressed growing concern that militants might attempt to detonate a device to disperse toxic chemicals and cause mass casualties.