(CNN) -- The killings of three people at Jewish sites in Kansas are a stark reminder that even as hate groups are on the decline in the United States, some among them are still deadly dangerous.
A leader of the Anti-Defamation League called Monday for institutions across the country to "use this horrible tragedy as a learning experience" and tighten their own security systems so that if "God forbid, this does happen, that the least ... harm can come to their people."
The ADL warned last week that violent attacks against Jewish targets could spike as the Passover holiday -- which begins Monday night and lasts eight days -- coincides with Hitler's birthday on April 20.
"Unfortunately throughout history, Jews have been a target around the holidays. We always feel it's necessary for Jewish organizations to try to ramp up security," the ADL's Evan Bernstein told CNN.
While anti-Semitic incidents in the United States declined last year, the number of violent incidents went up, according to an ADL study. Thirty-one reported assaults in 2013 included four men in Brooklyn attacking a Jewish man wearing a yarmulke; a group of girls throwing a bottle at a 12-year-old girl, with one of them calling her "dirty Jew"; and an attack on a man in Los Angeles by five men who yelled "Heil Hitler" before striking him, the ADL says.
The suspect in the Kansas shootings -- identified by police as Frazier Glenn Cross, who has also used the name Frazier Glenn Miller -- has long spoken openly about his hatred for Jews and called for "exterminating" them, according to the Southern Poverty Law Center, which tracks hate groups. The SPLC once sued him "for operating an illegal paramilitary organization and using intimidation tactics against African Americans," the center says.
The SPLC tracks groups that "have beliefs or practices that attack or malign an entire class of people, typically for their immutable characteristic."
"After four years of spectacular growth driven by the 2008 election of President Obama and the nearly simultaneous collapse of the economy, the radical right in America saw its first significant decrease in 2013," the SPLC says. The number of hate groups shrank last year for the second year in a row, down to 939, according to the center's calculations. These groups were demoralized by Obama's re-election, have been losing battles on issues such as same-sex marriage, and lost support when the "litany of disasters" they predicted -- such as the United Nations invading the United States -- didn't happen, the SPLC says.
"None of this is to suggest that the radical right in America does not remain highly dangerous," the SPLC warned in its report earlier this year. "The weakening of groups often has the effect of fostering, rather than retarding, followers' decisions to finally act out violently."
"Despite the decline, there are still enormous numbers of radical groups operating -- more than 2,000 of them, including hate and Patriot organizations. The single most important factor that has driven the growth of the radical right over the last five years, the ongoing demographic change to a non-white majority over the course of the next three decades, is still a source of enormous angst and rage for many."
Many of the hate groups the SPLC cites are anti-Semitic as well as racist. They include Christian Identity Groups, which espouse "a racist reading of the Bible that describes non-white people as lacking souls and Jews as the biological descendants of Satan," and neo-Nazi groups, among others. The SPLC also cites the New Black Panther Party, whose "white- and Jew-bashing lawyer Malik Zulu Shabazz relinquished his post," and was replaced by Hashim Nzinga, also "known for his anti-Semitism and anti-white racism."
The existence of these groups does not automatically suggest threats of violence. But the tragedy in Kansas "shows that one person with the mindset of that kind of hate can really damage a community in the deepest way," Bernstein said.
The SPLC's criteria for designating hate groups are a source of controversy. In 2012, the Family Research Council lashed back at the SPLC for designating it as a hate group.