With the Friday arrest of suspect Dzhokhar Tsarnaev, the Boston Marathon bombing investigation has entered a new phase. Yet to be uncovered are the motives of the bombers—the other suspect, Dzhokhar's older brother Tamerlan Tsarnaev, was killed during a gun battle with authorities—and their possible connection to foreign groups that use terrorism to advance political agendas.

To understand the Boston bombings within a larger context, National Geographic turned to terrorism expert David Schanzer, Director of the Triangle Center of Terrorism and Homeland Security at Duke University's Sanford School of Public Policy. (Related: Boston Suspects Put Chechnya in Spotlight)

Given the facts of this case so far, do you expect that this attack was the work of just these two individuals, or a larger conspiracy?

Intelligence officials will be working for weeks and months to determine if the Tsarnaevs had any linkages to individuals or organizations outside the United States. Domestic law enforcement will be focused on whether there were accomplices that aided or abetted the criminal conspiracy.

It is too early to tell what the answers will be. There are indications, however, that they were not part of a larger, sophisticated terrorist group.

The Tsarnaevs used a common construction for their weapon that was readily available on the internet and did not include liquid or plastic explosives, they made no effort to leave the country, they were poorly financed (otherwise why would they need to commit robbery), they did not target an event that would have meaning for an international audience, and they did nothing to publicize a political cause.

Does this case bear the hallmarks of religious/political terrorism? Do these bombers (or one of them) fit the jihadi profile, or something different?

There is no "profile" of a homegrown terrorist. The issue is whether the Tsarnaevs were motivated to violence by adoption of the al Qaeda ideology (I prefer not to bestow religious legitimacy on this ideology by giving it the label "jihadi" because this is an established Qur'anic principle).

The al Qaeda ideology is a mixture of religious, historical, political and economic themes that boil down to a claim that the West is at war with Muslims and must be confronted by violent means to enable Muslims and Islam to prosper. I believe it is certainly a possibility that one or both of the Tsarnaevs adopted this ideology and murdered innocent people in furtherance of it. We will need to learn more facts to make this determination.

In general terms, how would you characterize the threat of Islamist terrorism to Americans on U.S. soil?

I don't use the term Islamist terrorism. Islamism is a political movement that seeks a strong or dominant role for Islamic principles in the governing of a society. It has benign forms, but also highly dangerous extremist forms.

We have had numerous incidents of terrorism inside the United States inspired by al Qaeda and its ideology prior to and since 9/11. As the 9/11 attacks demonstrated, al Qaeda was a powerful and dangerous organization 12 years ago, but is now a shell of what it once was. Central al Qaeda and its affiliate organizations around the globe still aspire to execute attacks inside America, but their capabilities to do so are dramatically diminished. The threat is present, but no longer acute.

We also face the threat of homegrown terrorism, where individuals or small groups of Muslim Americans radicalize and engage in violence. About 20 Muslim Americans per year on average since 9/11 have been arrested for planning or perpetrating terrorist crimes. These individuals tend not to be highly skilled or capable terrorists, but as we saw in Boston, this does not mean that they cannot cause substantial harm.

Since 9/11, only eleven homegrown attacks have been successfully executed (including the Boston Marathon bombing), causing 21 deaths. The vast majority of these perpetrators have been apprehended before they could engage in violence. Homegrown terrorism is an ever-present threat, but even after this horrible week, it is a manageable one.

As we contemplate the Boston Marathon bombings, could you provide some statistical context for attacks of this type? On average, how many terrorist attacks occur, worldwide, in a given year?

The Global Terrorism Database operated by the START Center at the University of Maryland has the most comprehensive listing of terrorist incidents around the world from 1970 to 2011. The database identifies 104,689 attacks over the past 42 years. There were 5008 attacks in 2011 alone, an all-time high (but keep in mind that there is much more reporting of incidents globally now than 40 years ago).

How does the Boston attack compare, in terms of casualties, scale, etc., to other terrorist attacks around the world?

According to the database, there have only been 335 terrorist attacks since 1970 with over 100 casualties. The marathon bombing and the mayhem preceding the arrest of the final suspect caused 4 deaths and over 170 injuries, making it a significant and large mass casualty event. For some context, on average, there have been 3.3 casualties per domestic attack, and only 1.4 casualties per attack if 9/11 is excluded.

What is the trend today? Is terrorism being used less now than it was a few years ago, or are we just not hearing so much about it?

The decade since 9/11 has seen less terrorism (of all ideologies) than other recent decades. There were 168 attacks in the ten years after 9/11, but in the 1970s, there were 1357 attacks.

In the months after the 9/11 attacks, there was a general expectation-and dread-that 9/11 was just the first of many terrorist attacks inside the United States. Yet the total number of attacks since then is relatively few. Why is that, do you think?

The counterterrorism strategy against al Qaeda that has been executed since 9/11 has been extremely effective. We eliminated the safe haven that al Qaeda enjoyed in Afghanistan and captured or killed hundreds of senior leaders and thousands of rank and file militants.

It is also important that governments in countries like Saudi Arabia and Yemen, who were on the sidelines prior to 9/11, joined the fight because they felt threatened by al Qaeda as well.

We have also tightened our visa issuance process and border security (at a great cost to our international image and economy) so that it is much harder to enter the United States, especially from certain countries.

Some of the steps we have taken were counterproductive (the torture at Abu Ghraib prison, to name but one example). And we have not eliminated the sources of grievance at the United States that gave rise to al Qaeda and could spawn other terrorist movements in the future.

But we have crippled the organization that attacked us on 9/11 to the benefit of the United States and the world.

Much has been made of the "radicalization" of American Muslims—most prominently during the public hearings conducted by the House Homeland Security Committee in 2011-2012. What is your assessment of this phenomenon? And what, if anything, did we learn from those hearings that should be applied to the Boston attack?

Al Qaeda's ideology has been rejected by almost all Muslim Americans. Every major Muslim American organization in the United States has consistently and vociferously denounced acts of terrorism, by Muslims and non-Muslims alike.

Muslims Americans have cooperated with counterterrorism and law enforcement officials across the country. Studies show that a significant portion of the terrorism plots that have been thwarted are based on tips that have come from the Muslim American community. And many Muslim Americans are actively rebutting radical ideology within their communities, in their mosques, and on the internet.

We did not learn anything from the so-called radicalization hearings that we didn't know beforehand—very few Muslim Americans are attracted to this ideology and even fewer are inclined to engage in violence in furtherance of it. Homegrown terrorism is a real threat and can have devastating consequences, as it did in Boston, but it is not a widespread phenomenon.

Are there common themes, or lessons to be learned, in looking at past incidents (the Lackawanna Six, Maj Nidal Hasan, Faisal Shahzad) that involved Muslim terrorists who had "assimilated" successfully into American life? Looking back, are there patterns of behavior (social estrangement, for example) that should have been a red flag?

There is not one pathway to radicalization and so every case has its unique attributes. Efforts to identify a "profile" of a homegrown al Qaeda inspired terrorist have been unsuccessful.

Characterizing all homegrown terrorists as having "assimilated" is inaccurate. It appears that many homegrown terrorists have strong questions about their identity in America. Even if they go to an American school, are working in a decent job and live in a nice neighborhood, we can't assume that they are fully comfortable in American life and have reconciled the various strands of their identity, many of which may be in conflict.

One common strand is that many homegrown terrorists did not have a formal education and training in Islam. Their lack of this educational foundation makes them vulnerable to an ideology that claims to define the actions and beliefs of "good Muslims," but are actually contrary to the values and scripture of Islam.

As you think about the Tsarnaev brothers, could you comment about the role of psychological pressure (with an older and more radicalized Tamerlan leading the younger and less committed Dzhokhar) in such cases? Is this a fairly typical scenario in small-scale terrorist attacks? We saw something similar in the 2002 DC sniper case, with John Allen Muhammad and his follower Lee Boyd Malvo.

Counterterrorism officials and terrorism scholars will be analyzing the Tsarnaevs for many years once more facts about them become available. Groups dynamics have been proven to have a radicalizing influence on group members in many contexts, include terrorist organizations and cells.

The influence of loved ones can also be a powerful force toward radicalization, that can be especially influential on individuals with weak identities that feel isolated within a dominant culture. Yes, this case reminds me of the D.C. sniper case for many reasons (including the fact that it paralyzed a major city for three weeks, far longer than the Boston saga).

I also see similarities with the case in North Carolina with Daniel Boyd who radicalized his two sons—all three of whom are serving lengthy prison sentences.

We're aware of terrorism cases connected to the Arab world (Yemen, Saudi Arabia, etc.) But is there a precedent for radicalization of U.S.-based individuals connected to the Caucasus?

Not to my knowledge.

If the past is an indication, what effect will this dramatic, highly publicized case have on popular sentiment towards Muslims in the U.S.?

I wish it were otherwise, but this high profile case is likely to exacerbate many of the difficulties that Muslim Americans have faced since 9/11. Muslims face challenges from a variety of sources: official discrimination by government agencies, violent hate crimes against persons and property, blatantly prejudicial legislative efforts targeted at Muslim religious practices and subtle societal discrimination that impacts employment, housing, and other attributes of life in American.