How did the phrase "Never again" get associated with the Holocaust?

I often hear or read the phrase "Never again" used as a reference to the Holocaust. It is the motto of the Jewish Defense League and in the title of at least one historical account of the Holocaust. For what it's worth, the same reference was made in a South African political cartoon (Zapiro: April 19, 2002) about the Israel-Palestine conflict, and by the character Magneto at the climax of the most recent X-Men film. More recently, the phrase has been used to refer to genocide in general, but I'm pretty sure it started as a Holocaust reference.

My question is, what is the origin of this reference? Was it from a speech following World War II? A letter or prominent book? The best I've ever found is the article The Persistence of Genocide at the Hoover Institution of Stanford University, which opens:

According to the great historian of the Holocaust, Raul Hilberg, the phrase “Never Again” first appeared on handmade signs put up by inmates at Buchenwald in April, 1945, shortly after the camp had been liberated by U.S. forces.

Is this correct? Did use of the phrase spread from here?

I hope no offense is taken because of the potentially sensitive subject. I have asked here because it's a question about the etymology of a phrase.

It's associated with the Holocaust for this reason:

While the phrase “Never Again” was in use before World War II — it was the title of several early American films — it has, for Jews, become shorthand for an ongoing commitment to remembering the Holocaust. The phrase was first popularized as a Jewish rallying cry by militant Zionist leader Meir Kahane in his 1972 book “Never Again!: A Program for Survival.”

The book of a Zionist leader has made this phrase of special meaning to the Jews. The book was about the survival of the Holocaust attempt of the Jews.

The usage of "Never Again" might have first appeared on handmade signs put up by the inmates, but it didn't gain special meaning until Meir Kahane wrote his book.