It was profoundly reassuring for me that even after all we have seen -- Sept. 11, Oklahoma City and Columbine -- people reacted with such intense shock and horror at the attack on the U.S. Holocaust Memorial Museum and the killing of Special Officer Stephen Tyrone Johns.

There was disbelief and surprise: Why would anyone attack the Holocaust museum, a place dedicated to understanding?

Disbelief and surprise, however, were not among the reactions for the Anti-Defamation League or law enforcement. We had no illusions about what extremists are capable of doing. Make no mistake, neither police nor ADL was aware of a specific, imminent threat. But we know that right-wing extremists, above all among white supremacists and neo-Nazis, consider Holocaust denial a tenet of faith and see the museum as a concrete symbol of their hatred of Jews.

We also were aware that, whatever the specific reasons behind last week's attack, there is something percolating in the extremist firmament. Since the start of the year, there have been nine killings attributed by law enforcement authorities to extremists. These killings are the result of eight separate attacks, or aborted attacks, by extremists.

The most widely known example, was the murder of an abortion provider in Topeka, Kan., by an anti-abortion and anti-government extremist. In Little Rock, Ark., Abdulahakim Mujahid Muhammad shot two soldiers, killing one. Police believe he may have been planning attacks on Jewish institutions.

Three police officers were shot to death in Pittsburgh. The man charged with these murders was a virulent anti-Semite and racist involved in white supremacist online discussion forums. Police in Massachusetts arrested a white supremacist after he allegedly killed two people and raped and attempted to kill a third. The arrest pre-empted what authorities believed was a racially motivated killing spree directed at Jews and non-whites.

In May, four Muslim converts were arrested in New York for allegedly plotting to bomb two synagogues and shoot down planes at a military base. In Southern California, an animal rights extremist cell claimed responsibility for the car-bombing of a neuroscientist at the University of California at Los Angeles.

While these attacks are separate events, uncoordinated and unplanned, they reveal a shared belief on the part of a wide range of extremists in the legitimacy of killing and maiming to further the cause. In fact, these attacks are a chilling illustration of the range and diversity of the domestic terrorist threats facing our country that cover the spectrum from the far right to the far left.

Now is a good time to throw away assumptions about extremists and what they might try to do. Much about last week's attack was unusual. The alleged shooter was elderly, armed with an ancient .22 rifle and launching an attack on one of the city's most secure federal buildings. That the Holocaust museum had never before been the target of a serious incident only hammers home the obvious lesson for every Jewish communal institution. Vigilance and security are things we must take seriously and learn to live with.

Economic conditions, combined with anger over immigration and the election of the first African American president, may be contributing to the increased violence from right-wing extremists. Whatever the reasons, if there is a single lesson that must be absorbed by the Jewish community, the civil rights, Muslim, Hispanic, gay and lesbian communities, by any community or group targeted by extremists, it is that the days of thinking or hoping our institutions will be safe because we will not be noticed, or because these things simply do happen, are over.

The heroism of the museum's security personnel averted a greater tragedy. But it was the museum's professional leaders who must be applauded for having a plan in place to prevent and respond to every contingency, for devoting the resources to training its personnel, and for reopening after a day of mourning for Officer Johns to continue their mission of increasing understanding and fighting anti-Semitism and intolerance.

The Anti-Defamation League, founded in 1913, is the world's leading organization fighting anti-Semitism through programs and services that counteract hatred, prejudice and bigotry.