The online video's message was clear: Supporters of Islamic State who could not travel overseas to join the militant group should carry out attacks wherever they were in the United States or Europe.

Bangladeshi immigrant Akayed Ullah, 27, followed those instructions on Monday when he tried to set off a homemade bomb in one of New York's busiest commuter hubs, in an attack that illustrates the difficulty of stopping "do-it-yourself" attacks by radicals who act alone.

While harder to stop than attacks coordinated by multiple people — whose communications may be more easily monitored by law enforcement or intelligence agencies — they also tend to do less damage. Ullah was the person most seriously wounded when his bomb ignited but did not detonate in an underground passageway linking the Port Authority Bus Terminal and the Times Square subway station; three others sustained lesser injuries.