The threat from homegrown extremists to the U.S. homeland has been constrained in recent years by a variety of security measures. According to data collected by the New America Foundation, family members of extremists and members of the wider Muslim American community provided useful information in the investigations of about a third of the homegrown jihadist extremists indicted or killed since 9/11.
Noncommunity members provided useful reports of suspicious activity in another 9% of homegrown extremist cases, while an informant or undercover agent monitored almost half of all homegrown extremists.
The Boston Marathon bombing, however, demonstrate the potential for these security measures to fail when plotters have few, if any connections, to known terrorist groups.
Looking forward, a concern about the Boston attack is whether it represents an intelligence failure that could have been avoided through a better implementation of existing policies or a new trend where "lone wolf" extremism is largely undetectable with the existing systemic checks in place.
The threat from homegrown extremists has changed substantially in the past three years, becoming the province of fewer individuals who are less connected to foreign groups than was once the case.
Whether this remains the case will depend on law enforcement being able to successfully adapt to confront the threat from disaffected U.S.-based individuals motivated by al Qaeda's ideology as well as whether the past few years' counterterrorism successes against al Qaeda overseas can be maintained.