Since the Paris attacks in November, the authorities have been tracking every Islamist extremist in Brussels, raiding homes, tapping phones. How did ISIS pull the Brussels attacks off?

"I think it's a combination of how sophisticated these guys are ... [and] the very large number of individuals we are dealing with," said Michael Morell former number-two at the CIA and CBS News senior security contributor.

"The investigative report on Paris shows the use of throwaway phones, sophisticated document forgery, the making of bombs in Europe by bomb-making experts, the ability to move money around, the use of encrypted apps that intelligence services can't read," he continued.

To learn what else Morell had to say about fighting ISIS, watch the video above.