Saturday’s terrorist attack on the Marriott Hotel in Islamabad, Pakistan, caused massive destruction and loss of life. One of the reasons why: The bomb contained a lethal accelerant, found in some of the world’s most powerful munitions. The BBC coverage of the bombing notes:

There is no footage of the main blast because it destroyed the camera, but officials said the vehicle was packed with 600kg of high quality explosives as well as grenades and mortars. Aluminum powder was also used to accelerate the explosion and added to the ferocity of the blaze, officials said.

Aluminum powder has long been used to boost the power of explosives. Blast weapons like the 15,000-pound BLU-82 Daisy Cutter and the 21,600-pound "Mother of All Bombs" use it to increase their destructive force.

Devices with a high proportion of metal powder to explosive are termed "thermobaric." When the explosive goes off, the metal powder at the leading edge of the fireball burns as it contacts the air. With a crude device, the powder simply burns and adds to the fireball. In more advanced weapons, the burning metal produces a sub-sonic shockwave (known as deflagration); the most advanced produce a detonation (supersonic shockwave) of tremendous destructive power. I noted the potential risk from terrorist thermobaric devices back in 2004.

Normal, condensed explosives produce a very short pressure pulse. A "volumetric" one, from a detonating fireball, produces an extended blast pulse that is far more damaging to buildings. The

Marriott attack left a large crater, indicating that much of the blast came from a point source. The metal powder seems to have contributed only to the incendiary effects. According to the Guardian, "the temperature had reached 400C, investigators said, which made the hotel’s sprinkler system and the fire service useless."

By all accounts, there was a long delay before the device went off, with the truck burning sometime before the explosion. (See the video, above.) This was not a device built by master bombmakers. And the hotel’s security barrier performed a vital function of keeping the bomb away from the building. Distance is life in these situations. (Compare the Marriott blast with the Oklahoma City bombing; there was a lot of structural damage to the Alfred P. Murrah Federal Building, because

McVeigh was able to drive right up to it.) However, a thermobaric blast would extend the radius of effect of such a truck bomb significantly.

The blast and fire damage at the Islamabad Marriott were severe enough as it was. But a similar device with enhanced engineering could have leveled the building and caused far worse casualties. Terrorists showed that one of the most secure buildings in Islamabad was still vulnerable to attack, but there was far less damage than there might have been.