At approximately 2:54 p.m. on April 15, two apparent improvised explosives devices detonated near the finish line of the Boston Marathon, killing at least two and wounding dozens more. There are always large crowds near the finish line of a major marathon, and as one of the most internationally prominent marathons, the Boston Marathon is covered heavily in the U.S. and international media. This makes the event attractive to attackers.

At first glance, the devices used in the attack appear to have been small and may have been concealed in small bags or boxes. There are reports of three or four additional devices that did not explode. The size of the device can be judged by the lack of structural damage to the buildings adjacent to the blast sites. There were also only a limited number of broken windows in the area. The smoke seen after detonation was consistent with the results of a low-velocity improvised explosive mixture, perhaps something like flash powder or a sugar chlorate mixture. Based on the number of casualties and the types of wounds caused, it is likely that the attackers packed the devices with nails or other improvised shrapnel.

This type of device is rudimentary and can be built by nearly any actor. It was not particularly large or sophisticated. Small devices have been employed by jihadists in attacks such as the July 7, 2005, London attacks and by domestic terrorists such as Olympic Park bomber Eric Rudolph and the Unabomber, Theodore Kaczynski. At this point, it is too early to ascribe a motive or to identify a suspect.