MUNICH — As an expert in works of art that the Nazis called “degenerate” and in the dealers who traded them during World War II, Vanessa Voigt often wondered what had become of Cornelius Gurlitt, the son of a prominent Nazi-era art dealer and a figure she had come to view as a “phantom.”

Early last year, Ms. Voigt finally came face to face with the elusive man who kept popping up vaguely in her research. German customs officers had just stumbled on some 1,280 paintings and drawings — masterworks believed to be worth more than $1 billion — stashed in Mr. Gurlitt’s Munich apartment, and they turned to Ms. Voigt to help them understand what was going on.

As the customs officers confiscated the works, a distressed Mr. Gurlitt paced restlessly around his previously inviolable domain, muttering over and over to himself, “Now they are taking everything from me,” recalled Ms. Voigt, who was present. “He was mortified,” she said.

In an interview, his first, published on Sunday, Mr. Gurlitt, 80, told the German newsmagazine Der Spiegel that the confiscation of the artwork was a devastating blow — more difficult even than the loss of his sister, Benita, to cancer last year. “Saying goodbye to my pictures was the most painful of all,” he said....