I am interested in finding out US crime statistics like:  for murder,
what percentage of the victims knew their assailant?  The rest would
be anonymous attackers.  If this information is available for other
types of crimes that would be great too.

Hello fwscholl and thank you for your question.

Fortunately there are numerous sites that provide crime related
statistics.  I hope the below sites/information will meet you
requirements:

Bureau of Justice Statistics 
http://www.ojp.usdoj.gov/bjs/

FBI Crime statistics
http://www.fbi.gov/ucr/ucr.htm

DEA Crime statistics
http://www.usdoj.gov/dea/statistics.html

Intellectual Property Crime Statistics
http://www.cybercrime.gov/ipstats.htm


More specifically regarding crimes where the perpetrator was know to
the victim.........


===============================================

Homicides:
"Percent of homicides in which the offender was known to the victim,
1976-2000 aggregate"
http://www.ojp.usdoj.gov/bjs/homicide/teens.htm 

================================================

Identity theft:
"Only about 13% of victims who contacted the FTC indicated that they
personally knew the person who had stolen and misused their identity.
These relationships include family members(6%), friends, neighbors
(2%) and persons known to the victim in a similar capacity
(3%),roommates (1%), and personal associates from the victim's
workplace (1%)."
http://www.ftc.gov/bcp/workshops/idtheft/trends-update_2001.pdf

================================================

Sexual crimes:
"Women were more likely to report being raped or sexually
assaulted by friends or other acquaintances than by intimates,
relatives, or strangers.  Friends and acquaintances committed
about half of all rapes and sexual assaults (table 8). 
Intimates committed an additional 26%.  Altogether, offenders
known to the victim accounted for about three-quarters of all
rapes and sexual assaults against women.  Strangers committed
18% of such assaults."
http://www.ojp.usdoj.gov/bjs/pub/ascii/femvied.txt

================================================

Theft:
"Theft offenses were more likely to be reported by juveniles when the
incident took place at school than away from school (51% vs. 22%) or
involved a stranger rather than someone known to the victim (42% vs.
20%)."
http://www.ncjrs.org/html/ojjdp/2000_5_2/child_05.html

================================================

Stalking:
"The majority of cases of offline stalking involve stalking by someone
known to the victim prior to the stalking: 77% of women and 64% of men
had prior contact with their stalker."
http://www.cyber-stalking.net/about_cyberstalking4.htm

=================================================

Thank you for your question, and if you need any clarification of my
answer do not hesitate to ask.

Very best regards

THX1138

Search strategy included:
"crime statistics"  site:.gov
://www.google.com/search?q=+%22crime+statistics%22++site:.gov&hl=pt&lr=&ie=UTF-8&safe=off&as_qdr=all&start=30&sa=N