Perpetrator
Part of speech: noun
Definition: One who perpetrates; esp., one who commits an offence or crime.
Example sentence: The claim that too many criminals are being jailed, that there is over-incarceration, ignores an unfortunate fact: For the vast majority of crimes, a perpetrator is never identified or arrested, let alone prosecuted, convicted and jailed.
Culprit
Part of speech: noun
Definition: The person or thing at fault for a problem or crime.
Example sentence: While 'Visitation Street' has the markings of a traditional whodunnit mystery - starting with a missing girl, intrigue and many suspicious characters - Pochoda shows her hand early on by fingering a culprit. The book turns, then, into a 'whydunnit.'