c4 is a military grade plastic explosive. crucially, isis in sinai has been stockpiling c4 explosives. it's been using that explosive in a number of attacks in the sinai peninsula in recent months. so certainly consistent with a hypothesis that isis affiliates carried out this attack. they have claimed that they carried out this attack. >> so if it is someone who had access, they don't think it was a passenger who brought it through the security check point. who would that leave? airline or airport employees? >> they would be looking potentially at someone with access to the plane. people handling baggage, potentially security stuff, cleaners who might be able to