Help the science fiction writer complete this dramatic sentence for their latest novel.

By the time he was finally caught by the authorities, the eccentric billionaire was known _____ on a functional time machine for over a decade.

The correct answer is to have been working.

This sentence combines a past passive reporting verb (was known) with an action that was ongoing up until that past moment (for over a decade).

To express this continuous duration leading up to a past point, we must use the perfect continuous infinitive (to have been + -ing). "To be working" would just mean he was working at that exact moment, ignoring the "for over a decade" duration.