The purpose of the present article is to make an effective proposal for combating terrorist attacks related to illegal migration. It is stated that terrorist attacks and illegal immigration are organized criminal phenomena that are occasionally interconnected. The peculiarity of these crimes is that they radically affect societies and that combating preparatory illegal acts for their perpetration does not restrain the entire criminal activity. Under these circumstances it results that undercover operations are an effective, penetrative means of preventing organized criminal activities. Finally, it is concluded that the confluence of two particularly serious crimes leads to the proposal for adopting a new (more penetrative) type of undercover operation through the prism of the principle of proportionality.