folks from outside looking what's gone on before the event and of course, folks inside. >> and tell us more about these truck attacks because we have seen them take place in europe. we see how devastating they can be. explain how dangerous a weapon that is and a simple form but can do a lot of damage. >> so al qaeda has been putting out this magazine for years called "inspire magazine" and in it they have suggested means of attacks. one of them is the pressure cooker bomb we saw in the boston marathon bombing and in the chelsea attacks but also talked about using a truck to simply run down people and we've seen that in france and we saw it in canada and so this is certainly opportunistic attacks. they need to have control of the vehicle and be in a position to use it against unarmed vulnerable citizens and that's what you're taking off the table when you put out a perimeter of the sand trucks and controlling any type of vehicular traffic in the area. >> and police, of course, they only have a limited amount of