I was reading a sentence as:
...La polizia sospetta si tratti un gruppo di persone...
How could this sentence be rephrased? Is si tratti a formal phrase? If I am right, is there a spoken way for it?
The reason I think so is I usually read this kind of verb but haven't heard it in dialogues.
That phrase uses in an impersonal way the subjunctive mood of the verb "trattare"; but I think that the phrase is not completely correct. The correct phrase should be:
La polizia sospetta (che) si tratti di un gruppo di persone...
(The conjunction "che" can be omitted).
The verb "trattare" is used here with the meaning of "to be a question (of)"; "to be a matter (of)"; "to have to do (with)" see treccani.it/vocabolario/trattare point 4
The full sentence could help to find a good alternative, but you could as well use the verb essere ("to be"):
La polizia sospetta che (ci) sia un gruppo di persone...
If you prefer, you could use the expression to "to have to do (with)":
La polizia sospetta di avere a che fare con un gruppo di persone...
I would not recommend this expression, especially for writing, because it is a bit too verbose. I think that even the usage of "trattare" would be discouraged for example on a newspaper article.
...La polizia sospetta che un gruppo di persone sia coinvolto...
...La polizia sospetta che un gruppo di persone sia responsabile...
