Just trust me on this...
The military had all the intelligence/evidence it needed to clearly demonstrate that both Adams and McGuinness were active members of the Provisional IRA Council in the early 1980's when they were professing to be "political". They were directing PIRA policy so the blood is on their hands just the same as it is on those who committed the actual acts. The problem was that arresting them would have been politically "awkward" because we couldn't be seen to be arresting political "leaders".