Expanding my thinking…Strzok and Page referred to Comey as the D.
So by that, D5 could equal the 5th Director of:
1) the Bureau of Investigation (1908–1935)
2) the FBI (1935–present)
3) Central Intelligence (1946–2005)
4) the CIA (2005–present)
Pics related, respectively.
What ever way you cut it, the 5th Director of each was key to the mess we're in now.