An interesting link I stumbled across that I would think should have been posted here by now, but I can't find a match at qresear.ch.
FBI's 'Clean' Teams Follow 'Dirty' Spy Work =
By Roberto Suro
Washington Post Staff Writer
Monday, August 16, 1999; Page A13
In FBI slang they are known as "dirty teams" and "clean teams," or as "dark" and "light" agents, or even more cryptically as "fives" and "sixes." The two groups are deployed together when terrorists strike or when top-secret information has gone astray, and they often spend months, even years, working in tandem. Yet they rarely talk to each other.
As the FBI becomes more and more involved in overseas investigations of terrorist threats, using two distinct teams of agents kept apart by an imaginary wall has become a key to separating criminal cases that can be prosecuted in open court from intelligence secrets that must be protected forever.
On one side of the wall are agents privy to top-secret intelligence gleaned from spy satellites or foreign officials whose loyalties and methods may not be fit for public disclosure. On the other side are agents protected from such information so that when they are challenged in court, they need not fear revealing national security secrets or introducing evidence tainted by human rights abuses committed in a faraway jail.
"We find ourselves more and more frequently in situations that require us to protect intelligence assets even as we develop evidence that can be used in a criminal prosecution," Larry R. Parkinson, general counsel of the FBI, said in an interview.
Whether the target is Osama bin Laden, the terrorist chieftain accused in last August's African embassy bombings, or a campaign fund-raiser suspected of working for the Chinese government, this division of labor–once limited to Cold War espionage cases–is becoming a more common feature of federal law enforcement. Following a presidential directive issued last May, the FBI now works intimately with intelligence agencies and the military in foreign counterterrorism operations.
Caps and PDF attached (archived as well).
Sauce:
https://www.washingtonpost.com/wp-srv/national/daily/aug99/dirty16.htm