Ah but there is such a thing as criminal negligence, reckless endangerment and those are criminal charges with criminal penalties. Intent to cause injury or death is not a requirement. Simliarly, some "accidents" simply wouldn't have happened if the person responsible didn't knowingly do something wrong or dangerous to cause the accident, although that person did not intend harm to anyone. Those are the kinds of cases that could result in criminal penalties.
And to Hus: As for your sister's comment about witnesses, that really isn't relevant to the sentencing. Usually there are sentencing guidelines. Whether there are witnesses or no witnesses may or may not be relevant to the case as evidence for a conviction depending upon the circumstances and other evidence in the case.
Here are some links on involuntary manslaughter:
A recent case in the news: http://www.lasvegassun.com/sunbin/stories/nat-gen/2003/jun/10/061006523.html
Definition of: http://www.lectlaw.com/def2/m011.htm
Model Jury Instructions: http://www.socialaw.com/Rules/homicide/involmanslaughter.html