No correct answers yet. Here's today's clue.
Looking for an aircraft.
1. The military version of a civilian aircraft. Unlike most civil-to-military conversions, the military version was larger than the civilian version.
2. And a later variant of the military aircraft was even larger.
3. Used by the militaries of several countries, and also by some police forces.
4. One of those police forces was the FBI, during an incident that got a lot of publicity.