Via The Intercept:
The Transportation Security Administration said it is unlikely to detect and unable to extinguish what an FBI report called “the greatest potential incendiary threat to aviation,” according to a classified document obtained by The Intercept. Yet despite that warning, sources said TSA is not adequately preparing to respond to the threat.
Thermite — a mixture of rust and aluminum powder — could be used against a commercial aircraft, TSA warned in a Dec. 2014 document, marked secret [PDF here]. “The ignition of a thermite-based incendiary device on an aircraft at altitude could result in catastrophic damage and the death of every person onboard,” the advisory said.
TSA said it is unlikely to spot an easy-to-assemble thermite-based incendiary device during security screening procedures, and the use of currently available extinguishers carried on aircrafts would create a violent reaction. The TSA warning is based on FBI testing done in 2011, and a subsequent report.
A thermite device, though difficult to ignite, would “produce toxic gasses, which can act as nerve poison, as well as a thick black smoke that will significantly inhibit any potential for in-flight safety officers to address the burn.”