That's covered in the link I showed.
The breakdown includes the 44% of murders (not the 80-90% figure you suggest) where the relationship between killer and victim is unknown:
- 13.5% family members
- 12.4% stranger
- 29.7% other known
- 44.0% relationship not known
The family members figure could include street gang deaths if one family member is killed by another family member as part of street gang activities but the largest number is husband/wife, wife/husband relationships which, together with parent/child, child/parent are unlikely to be street gang related.
edited to add.....
repeat of the link for those who are interested:
http://www.fbi.gov/about-us/cjis/uc...o-law-enforcement/expanded/expandhomicidemain
the expanded tables are even more interesting and informative - thank you FBI
http://www.fbi.gov/about-us/cjis/uc...o-law-enforcement/expanded/expandhomicidemain