Exactly what hit the Pentagon on September 11th 2001? A Boeing 757-200 - or a cruise missile? Meyssan investigates and, with the aid of official colour photographs, suggests it was not American Airlines flight 77. Detonation or deflagration? Explosive materials are divided into two groups: explosives create a shockwave whose speed of propagation is greater than 6000 feet per second - they "detonate". Explosive materials, whose shockwave speed is lower, do not detonate, they deflagrate. According to Meyssan, the image of the impact on the Pentagon is very instructive in regard to the nature of the explosion. The speed of propagation on the shockwave is very high. The colour of the flame is consistent with an explosive of high energetic power. The explosion does not correspond to a deflagration of kerosene (airline fuel). Nor does the exit hot of the alleged plane - which pierced three separate buildings - add up.