Last Monday in Quebec, a man who had converted to Islam and had become radicalized purposely crashed his car into two soldiers, killing one of them, before he was shot dead.

　　Two days later, another gunman with a similar story killed a soldier standing guard at a war memorial in Ottawa. He later raced into the Parliament building, before being shot dead.

　　Then Thursday, in Queens, New York, a man who had posted comments sympathetic to the jihadists used a hatchet to attack four rookie police officers posing for a picture on the street. He, too, was shot dead.

　　Today, on the Sunday talk shows, the heads of the House and Senate Intelligence Committees commented on the attacks.

　　CHARLIE ROSE: What kind of threat does that pose to our own national security?

　　REP. MIKE ROGERS: Huge, and getting worse.

　　SEN. DIANNE FEINSTEIN: These attacks and the multiplicity of attacks in 2014 show that their propaganda is having some effect.

　　HARI SREENIVASAN: For more about all this, we are joined now from Boston by Jytte Klausen. She is a professor at Brandeis University and the Founder of the Western Jihadism Project, which tracks the activity of Islamist extremists in the West.

　　So, I want to ask, what is behind these lone wolf attacks?

　　JYTTE KLAUSEN: Well, we call them lone wolves, but, in most cases, they have been connected to networks and peer groups and militants for some time.

　　And they carry out the attacks by themselves, but they are not actually lone wolves, in the sense that they had just become radicalized off the Internet or something like that. It — of course, there are exceptions to this general rule.

　　But, right now, there is a call out from the Islamic State group, sometimes referred to as ISIL, to carry out attacks on — on people who represent the Western states.

　　HARI SREENIVASAN: OK. So we are also hearing more frequently about the sympathizers in the West who are lured to go and fight for ISIS.

　　What are the reasons for that?

　　JYTTE KLAUSEN: Well, one reason is that have been able to go.

　　ISIS, or ISIL, has invited them. There were many Westerners who tried to go and fight for al-Qaida in Iraq in the previous insurgency in Iraq, and they weren't welcome. But ISIL has been pursuing a colonization strategy in Syria for some time.

　　And so, in the course of 2013, they started inviting Westerners to come and settle.

　　And there were many Westerners who thought it was a very attractive proposition to walk around the streets of the — the Syrian cities that they refer to as the liberated zones and police the local Muslim population in those places.

　　So they were very attracted to the idea of getting control and being the big man or big woman on the block.

　　HARI SREENIVASAN: OK.

　　Well, once they get there, the story might change a bit. As your research tracks, there is actually a much higher mortality ratio of the Westerners who go in there.

　　Oftentimes, they are used as suicide bombers, because they are not really much more good to ISIL or ISIS?

　　JYTTE KLAUSEN: Yes, that is correct.

　　But, in 2013, the mortality rates weren't so high. They have really picked up since the start of this year.

　　And we are now picking up evidence of people who want to come home and have had regrets. But, at the same time, even as that is happening, there are also new people who are leaving.

　　So, there are — by my count, based on estimates from the different Western governments, there have been around 3,000 Westerners who have, at one point in time, gone off and joined the extremist jihadist groups.

　　HARI SREENIVASAN: What happens when they come back? Is there evidence to show that they are more likely to launch an attack at their home in the West?

　　JYTTE KLAUSEN: There is evidence of that.

　　But there are also some coming back who are exhibiting signs of having had regrets, particularly amongst some of the younger groups, the women, some of the teenagers who had taken off.

　　But they are by no means the only stream that has gone off. There are very hardened folks who have gone off, to people who have been experienced from previous insurgencies.

　　A Boston man named Ahmad Abousamra, who went both to Pakistan and to Yemen and took off to Syria — to Syria in 2006, is now believed to be in charge of the social media operations on behalf of ISIL.

　　And so we should be careful not to draw too fast conclusions about what sort of threat these people present when they come back.

　　And, for sure, we know, from previous experiences with insurgencies and Westerners going off, that having had the experience of learning how to carry out violence, shooting a gun and putting together a bomb, they will come back, and they will try to carry out violence here.

　　And there have been incidents already that have fortunately been foiled, with the exception of one…

　　HARI SREENIVASAN: Right.

　　JYTTE KLAUSEN: … an incident in Brussels at the Jewish Museum there.

　　HARI SREENIVASAN: All right. Jytte Klausen from Brandeis University joining us from Boston today, thanks so much.

　　JYTTE KLAUSEN: Thank you.