JUDY WOODRUFF:

The New York Times reported late today that one of the brothers being hunted for the Paris attack received training at an al-Qaida training camp in Yemen. This is according to a senior American official.

The brothers, who were born and raised in France, had a secular Muslim upbringing before their apparent radicalization. There have, of course, been other attacks on the continent, and thousands of European Muslim extremists have traveled to Iraq and Syria to fight.

To find out more about what's leading to the radicalization of many of these young men, I spoke earlier this afternoon to Peter Neumann, director of The International Center for the Study of Radicalization. He's at King's College.

Peter Neumann, thank you for talking with us.

First of all, what do you think is most important for us to know about this attack, about these two brothers in terms of understanding what went into this, what was behind it?