Consider an earthlike world where the following happened:
An UFO "crashed" somewhere on Earth some yrs ago.
It sunk into a swamp and was never found (since the swamp was huge and the UFO was small).
Recently, one found a "human" who survived damage that would have been lethal to a normal human.
One smuggled some persons into its social environment who tried to fool it so he tells them his "secrets", but they found nothing, it seems to be inable to talk about private stuff.
One analyzed it, and one found out that its body and its mind were just like normal humans (regarding the capabilities and structure).
Anyway, after several other experiments, that did not answer anything, one tried to kill it, but it survived. (severely but not lethally wounded)
After that, one got interested in what was going on, so one continued with the experiments and one found out that something in its body regenerated the lethal damage each time one tried to kill it.
since one suspected a connction between the UFO and the strange human, one uncovered the ufo.
When one opened it, one found 12 vessels.
Eleven contained nothing (anymore), and the twelfth contained some greyish stuff.
When one analyzed it, one found out that the greyish stuff were nanorobots.
due to some experiments, one found out that the nanorobots were able to form some sort of film and move quite fast as such.
The nanorobots could have gone anywhere on the world (they are fast when on their way as film).
One is powerful enough to interview/analyze millions of humans.
due to further cruel experiments, one found out that these nanobots cause unborn 0th-month "childs" to become nearly immortal "minions" with a certain "mission". (one inflicted small childs with corrupted nanobots (corrupted by radiation, causing them to "contain" corrupted missions), causing them (children) to show some behaviour (doing "mission") that lead to this knowledge.)
since there where 12 vessels, and, usually, if no one splits up the nanorobots out of one vessel they form a single film, one came to the conclusion that there must be 12 "minions" out there (in any country), all at roughly the same age (about one month birth time span).
while the "minions" are not lethally injured, there is no way to detect the nanorobots in the body.
when injured lethally, all nanorobots that are not broken go out of their hidings and regenerate the body as much as necessary for survival.
The "mission" a minion has is more or less its true identity, every "minion" has knowledge and ethical values adapted to its "mission".
Part of every "minions" mission is to play a raw human role, consisting of stereotypical interaction with their family and stereotypical behaviour at work and to avoid unnecessary communication with humans and other "minions" (any communication due to business or stereotypical family life is necessary).
Because a "minion" is a human, it acts "ethical" (like its own ethical values suggest) most of the time.
A "minion" will however never talk (or write etc) about its mission.
A "minion" may lie.
A "minion" does not know that it is a "minion" and immortal.
And here are the questions:
can a suspect prove the agency that he's not a "minion" without dying?
can one/how do one find all 11 missing "minions" using knowledge and resources roughly equal to that agencies knowledge and resources?
can one/how do one find out whats a "minions" "mission" without waiting for it to start doing its "mission" using knowledge and resources roughly equal to that agencies knowledge and resources?
How do one solve the problems above with as little law-breaking (english law) as possible? (law breaking that can't be discovered also counts as law breaking)