The slaying of Trayvon Martin in Florida and the recent shooting spree of five blacks in Oklahoma are part of a broader, but little-noticed, change in how Americans kill each other.
The rate of interracial murder is growing.
Over the past 30 years, black-on-white murders rose from 6 percent of all homicides to 8 percent, according to a study of 550,000 homicide reports assembled by Scripps Howard News Service from FBI and local police reports. White-on-black killings rose from 3 to 4 percent. All other racial combinations rose from 3 to 5 percent.
"There haven't been a lot of studies done on this. But we do have racial groups mixing together more than they used to, so it makes sense that there would be more homicides across race," said University of Florida Law School professor Katheryn Russell-Brown, author of "The Color of Crime."
The overwhelming majority of homicides are still intra-racial -- whites killing whites or blacks killing blacks. But these killings have fallen from 88 percent of all reported violent deaths in the 1980s to 83 percent from 2000 to 2009.
The rise in the percentage of interracial homicides has occurred while the total number of homicides has fallen dramatically in recent years.
Intra-racial killings have plummeted. White-on-white slayings tumbled from 63,704 deaths in the 1980s to 46,179 deaths in the 10-year period from 2000 to 2009. Black-on-black killings dropped from 56,931 to 41,457 deaths during the same periods.
Black-on-white killings grew from 8,503 to 8,530, while white-on-black slayings dropped from 4,745 to 4,380. Homicides involving all other racial combinations, including Asians and American Indians, rose from 3,854 to 4,981.
These black-on-white or white-on-black killings are especially likely to occur in large urban states like Florida, Massachusetts, New Jersey and New York. Contrary to popular stereotypes, interracial killings are relatively rare in rural Deep South states, occurring at a rate well below the national average.
Several crime experts agreed this rise reflects increasing social contact between Americans of different races occurring in many, but not all, communities.
"Both white-on-black homicide and black-on-white homicide increase slightly as the percentage of blacks in the population increase," said Dallas Drake, principal researcher at the nonprofit Center for Homicide Research in Minneapolis, after analyzing the Scripps data. "The more integrated a community is, the more likely there will be cross-race homicide occurring."
Public attention on interracial homicides has intensified following white-on-black killings in Florida and Oklahoma:
-- Martin, a black teenager, was shot and killed by neighborhood watch volunteer George Zimmerman Feb. 26 as he walked through a gated community in Sanford, Fla.
-- A white man, Alvin Watts, and a Cherokee Indian, Jake England, were arrested in Tulsa, Okla., following an April 9 shooting spree that left three black people dead and two others wounded.
Interracial killings have been rising in both states. There were 73 white-on-black homicides in Oklahoma in the 1980s and 81 from 2000-09. Black-on-white homicides rose from 81 to 138 during the same period.
Florida's white-on-black killings grew from 338 in the 1980s to 414 from 2000-09. Black-on-white homicides rose from 529 to 858 during the same period.
The rate of white-on-black killing varies significantly in America.
The Scripps study found that the greatest incidence of white-on-black killings, as a percentage of all homicides, are found in New York, Connecticut, Rhode Island, Nevada, Massachusetts, Florida, New Jersey, Delaware, California and Maryland.
But states like Alabama, Georgia, Mississippi and South Carolina all have white-on-black murder rates well below the national average, even though these states have large black populations.
Similar variances occur in black-on-white homicides.
Crime experts said this probably reflects the rate at which white people and black people interact.
"Some cities are very segregated and blacks and whites have very little contact. Other cities are much less segregated and there is black and white contact at a greater rate," said criminologist Charis Kubrin at the University of California, Irvine.
"We found that areas with high levels of racial interaction had higher financially motivated killings," said Kubrin, who has studied motivations for interracial homicide. "Interracial killings rise and fall relative to opportunities for intergroup contact."
The study found that the percentage of interracial killings among strangers is decreasing. In the 1980s, about 47 percent of white-on-black killings occurred between people who were strangers. That figure dropped to 40 percent since 2000.
The change was even more pronounced in cases of black-on-white homicides, where 48 percent were strangers in the 1980s, and only 34 percent in recent years.
The study was based upon 555,179 homicide reports gathered
by Scripps Howard News Service. Most were provided by the FBI, although Scripps collected 17,648 homicide reports that were never reported to the federal government from police in Florida and the District of Columbia.