Nationwide, hate group membership has dwindled, but the number of groups has grown, said Jack Levin, professor of sociology and criminology at Northeastern University in Boston and an author of several books on hate crimes.
The old guard groups have undergone a "crisis of leadership," Levin said, as some leaders died and others were locked up.
Ku Klux Klan meetings were regular occurrences in Cecil County as recently as the mid-1990s and members marched on the Annapolis State House in 1994 and 1998. And as recently as 2006, the Klan held a rally at the Antietam National Battlefield in Sharpsburg.
But those occasional rallies belie the group's actual numbers. These days, the Klan is down to about 5,000 members nationwide, according to the Southern Poverty Law Center, while neo-Nazi and other white supremacist groups have gained prominence.
It's unclear how the growth of the organized groups correlates to violence committed by lone extremists, or whether either have much to do with the vandalism or graffiti that turns up occasionally at synagogues or in black neighborhoods. Levin contends that relatively few hate crimes - no more than 5 percent, by his estimate - are committed by people who actually are members of organized groups.
"On the other hand, the most dangerous and most hideous of the crimes are often committed by the members of these groups," he said. "They may be small in number, but they're more likely to commit murder, and assault and armed robbery."
Such violence is rare in Maryland, although recent incidents have shaken minority communities.
In 2004, a Charles County arson - among the largest residential arsons in state history - ignited racial tensions. Twenty-seven new homes in an upscale subdivision were set ablaze, causing $10 million in property losses. One of the young white men convicted in the case said he had been angered by the prospect of black homeowners moving in.
The Justice Department's civil rights division did not find enough evidence of racism to warrant hate crime charges, but 32 area residents filed a civil rights lawsuit that is pending.
Arthur C. Abramson, executive director of the Baltimore Jewish Council, said he thinks anti-Semitic incidents in Baltimore are rare.
The handful of incidents that occur each year involve hateful graffiti or threats against individuals that are never carried out, he said.
Still, he said, the Holocaust Museum shooting should serve as a reminder that "we can't let our guard down."
