Formal:
* Accused
* Supposed
* Presumed
* Reported
* Rumored
* Said to be
Informal:
* Said
* Thought to be
* Believed to be
* Suspected
Other options:
* Purported
* Hypothetical
* Contested
* Disputed
Examples:
* Formal: The alleged perpetrator was apprehended by police.
* Informal: The guy said to be the thief was caught on camera.
* Other: The purported author of the manuscript remains unknown.
It's important to choose the synonym that best reflects the specific situation and the degree of certainty you want to express.