Shooting at Holocaust Museum in Washington, D.C.

Police gather in front of the museum after the shooting. (Win McNamee / Getty Images / June 10, 2009)

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DENVER, Colo. - The national authority on hate groups actually names 15 organizations they say are active in the state.

Most have been around for a while. So why is there an outbreak of violence now?

At the Holocaust Memorial in Washington DC, a monument to unspeakable crime, it became the scene of an unimaginable shooting.

It proved exactly what hate crime experts have been thinking all along.

"When hate is not exposed, when it goes unchecked, it can lead to violence," Anti-Defamation League Director Bruce DeBoskey told us.

From Wichita, Kansas, where an anti-abortion fanatic assassinated a doctor, to New York where globalization foes blew up a Starbucks, to Arkansas where there was a fatal shooting at an Army recruiting center, extremist groups have been engaging in extreme violence.

The Anti-Defamation League says there's a reason it's happening now.

"History has taught us, that when the economy goes down, bigotry goes up," DeBoskey says. "They look for people to blame, and often times that's when hatred and bigotry come into play."

To hear that is one thing. But today we saw it for ourselves…when an e-mail came to the station

Subject line: "Holocaust Memorial Shooting."

"There are a million powder kegs ready to explode ..." a Denver man wrote. "And I am one of those potential powder kegs."

"Still discouraged, still unemployed despite a master's degree…and no one is listening."

"There will be millions upon millions of powder kegs. And many are more violent than I am."

We showed the email to the Anti-defamation League, and they were greatly concerned.

The only solution for all of us, they say, is to speak out before it's too late.

"Because by shining the light on hate … it helps reduce the acts of violence that stems from hate," DeBoskey said.

The man who e-mailed us said he was saddened by the Holocaust shooting, and was not violent. Still experts we talked to said the letter needed to be handed over to the appropriate authorities.