segregated. essentially allowed to meet freely, to plot. and they had an ingenious way of communicating. >> they were able to write their contact details on the white elastic of their boxer shorts, prison-issued boxer shorts. they networked. they had phone numbers, details of fathers, villages, uncles, whatev whatever. >> reporter: he said it was the incubator of isis. if there was no american prison in iraq, there would be no islamic state now. >> most of the senior leadership and probably a good portion of the mid level management and foot soldiers came from bucca. because tens of thousands of people were held there over the years. and so just when they got out, they had little to do and they had these established networks. and it's clear that they had done their homework in the prison. >> reporter: and as he left, according to a former camp commander, abu bakr al-baghdadi