well, crap! i pasted the one without 'unknown' category. Sorry about that Jordy! Coz I lurv you so much, I'll just point you at this one among lover's triangle murder, 10 were committed by people who had an "unknown" relationship to the victim. :)
Circumstances: Acquaintance Unknown
Felony type total: 3217 6015
Rape 11 17
Robbery 221 432
Burglary 21 26
Larceny-theft 2 5
Motor vehicle theft 1 6
Arson 9 21
Prostitution and 3 3
commercialized vice
Other sex offenses 3 1
Narcotic drug laws 245 315
Gambling 2 2
Other - not specified 68 130
Suspected felony type 57
Other than felony type total:
Romantic triangle 57 10
Child killed by babysitter 23 1
Brawl due to influence 68 12
of alcohol
Brawl due to influence 36 25
of narcotics
Argument over money or
property 104 40
Other arguments 1154 543
Gangland killings 23 30
Juvenile gang killings 21 489
Institutional killings 9 2
Sniper attack 3
Other - not specified 446 460