By Karl Vick
June 4, 2015

Correction appended, June 4

The fatal shooting of Usaama Rahim on a Boston street on June 2 after he lunged at officers with a knife illustrates only too vividly the increasingly atomized nature of the domestic terrorism threat.

Long gone is the command-and-control hierarchy of al-Qaeda. Now investigators must scan for threats that can pop up wherever social media connects extremist doctrine with determined individuals.

Rahim, 26, who had written online that he intended to kill a cop, was reportedly plotting an unspecified attack with others, one of whom was in custody. Such a plan would mark a dangerous new development. Up to now, the domestic attacks inspired by ISIS have largely been impulsive.

The group appears chiefly intent on gaining ground in Syria and Iraq. But terrorism experts warn that it will inevitably train its fire on Western targets. And with 46,000 sympathetic Twitter accounts worldwide, according to a Brookings Institution study, distinguishing pretenders from credible threats will be a daunting challenge.

–KARL VICK

Correction: The original version of this story incorrectly identified the terror suspect killed by police in Boston on June 2. He was Usaama Rahim.

Contact us at editors@time.com.

This appears in the June 15, 2015 issue of TIME.

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