A truck attack on a Christmas market in Berlin, Germany left 12 dead and injured scores more Monday. German Chancellor Angela Merkel has called it an act of terror. ISIS has now taken responsibility.

Authorities had one suspect in custody, but that suspect has since been released. The killer is still at large.

For more, Take Two spoke to Erroll Southers, director of Homegrown Violent Extremism Studies at USC.

To listen to the full interview, click on the blue media player above.