When Henry Mayer, chief archivist at the U.S. Holocaust Memorial Museum in Washington, first called the FBI about Alfred Rosenberg's missing diary, Robert K. Wittman, head of the art-crime team in Philadelphia, had only the broadest sense of whom Mayer was talking about.

"I'd heard of [Rosenberg], and I had an idea that he was a high-ranking Nazi," Wittman recalled the other day.

Mayer wanted the diary - badly.

"I knew it had a lot of information," Wittman said. "Nowhere is it written down, anywhere, that Hitler ordered the killing of the Jews. There is no diary, no journal or memo that mentions extermination. We thought this could be it." It took more than a decade to track down and recover Rosenberg's secret musings.

Now, Wittman, working with Pulitzer Prizing-winning author and journalist David Kinney, has written The Devil's Diary - the story of the diary's recovery and the deadly epic story it illuminates.

Rosenberg might not be as well-known as Goebbels and Himmler, but he served as the "chief intellectual" of the Third Reich, tirelessly formulating and promulgating its anti-Semitic message, which was largely his own.

"The commandant at Auschwitz, Rudolph Höss, said what allowed him to do these mass killings was that he had read the writings of Rosenberg, Hitler, and Goebbels," said Wittman. "Höss was a regular colonel put in charge of this camp who was killing millions of people. And what allowed him ethically and morally to do that was the writing of Rosenberg, Hitler, and Goebbels. He said, 'I read it, and I believed it.'

"It's really terrifying."

The pursuit of this grim material led Wittman into the baroque world of flamboyant lawyer Robert Kempner, the late Nuremberg prosecutor and onetime Lansdowne resident, and Kempner's former lovers. All were brought together by an immense saga of megalomania fused with anti-Semitism.

Beyond his lurid philosophizing, Rosenberg oversaw the looting of art, books, archives, and just about everything else. He supervised activities in the occupied territories east of Germany, including death camps, slave-labor programs, starvation policies, and unimaginable theft and destruction.

As chief theorist, he crafted Germany's completely ideological educational system, ensuring that when the first generation of Nazis passed on, other indoctrinated generations would follow.

Rosenberg was with Hitler from the beginning of the Nazi Party after World War I, and although he did not record the führer's advocating mass murder, Rosenberg's diary provides deep insight into one of the darkest periods in human history.

It shines light on everything except the scrawled words Kill them all.

"There's definitely a total lack of empathy," says Kinney. "He's such a cold figure. There's no sense the policies he's been engaged with have any impact on real human beings. When he complains about the forced-labor program, he complains that it's harming Germany's reputation in the occupied territories. He's not worried about the millions of people being torn out of their lives and taken into slavery."

The 500-page diary covers the period from 1934 until Germany's defeat in 1945. Along with tons of other documents, it was impounded at war's end and served to underpin the immediate prosecution of Rosenberg and 21 other Nazi leaders for war crimes.

One of the American prosecutors was Kempner, a German-born lawyer who had escaped the country in 1935, eventually making his way to the Philadelphia area with his wife and also a lover/secretary.

Kempner, a Jew, assisted in numerous cases at Nuremberg. After Rosenberg was hanged Oct. 16, 1946, Kempner stayed in Germany. As a Nuremberg prosecutor, he had gathered millions of Nazi documents, including Rosenberg's four-ton archive. But instead of returning them to the Allies' document centers as mandated, Kempner shipped all of this material to Lansdowne, where his wife and former lover were ensconced.

Meanwhile, Kempner had corralled another secretary/lover in Germany. He never returned to live in the United States, but he kept up a ceaseless and loving correspondence with those in Lansdowne.

When Kempner died in Frankfort in 1993 (his wife had died in 1982), he did so in the arms of his two surviving lovers. The Rosenberg papers remained in Lansdowne.

A few years after Kempner's death, one of his former lovers contacted the Holocaust Museum and asked whether it would be interested in the Kempner archive.

Mayer, the archivist, eventually took a look in 1997 and found the Kempner house literally stuffed, basement to attic. The archivist quickly determined the Kempner papers were a gold mine, and Kempner's estate bequeathed the archive to the museum.

It would take several years of legal wrangling to get them out of the Lansdowne house.

In the meantime, one of the former lovers contacted Herbert Richardson, a minister who ran a small publishing house in Lewiston, N.Y. She asked for his help and advice on the papers.

Richardson was extremely sympathetic and quickly established the Robert Kempner Collegium in Lewiston for archiving and housing the immense trove. Some of the documents found their way to Lewiston.

In 2000, Mayer called the FBI, and Wittman entered the case. Some documents were seized in Philadelphia and held until the courts could sort out ownership.

But there was no diary in evidence.

Wittman was stymied. He eventually retired in 2008 and established his own security consultancy. Then he heard from Mayer again.

"Basically, it's a last-ditch effort," Wittman recalled. With the assistance of some old government colleagues, Wittman saw to it that Richardson, who lived over the border in Canada, was stopped and searched while entering New York state. His crossings were logged.

Finally, federal agents hit Richardson with a subpoena, and in spring 2013, the pastor-publisher produced the diary, which can now be seen online at the Holocaust Museum website, www.ushmm.org.

"Here's the thing," said Wittman. "People steal artwork. They steal documents. I've recovered an original copy of the Bill of Rights. . . . I've recovered a first folio Shakespeare. . . . With those books, we know what's in them. This we didn't know. This was priceless. The value was the information. To be able to stare into the mind of this dark soul, Rosenberg - that's what the value of this was. You can't put a financial value on . . . new information that helps us understand the greatest cataclysm of the 20th century. That's where this was so important - and why it was important for the Holocaust Museum to get it back and why we chased it for 13 years."