With 22 players, my estimate would be 2 mafia, possibly a third - or unaffiliated serial killer, a doctor (shielder), and a detective.

If the mafia guys didn't start out knowing each other, that may explain the lack of a kill - maybe they did investigations instead, trying to find their twin (or the detective, to get rid of him).

If that's so, it marks them as cautious players, which (making a leap) also might mark them as seasoned players.

Not much to go on, but it's early times.