A reproduction of a self-portrait by German painter Otto Dix presented during a news conference on Nov. 5, 2013 in Augsburg, Germany, on the discovery of nearly 1,500 paintings in an apartment belonging to Cornelius Gurlitt. (CHRISTOF STACHE/AFP/Getty Images)

Until six months ago, almost no one, even in his native Germany, had heard of Cornelius Gurlitt. Shy and reclusive, he lived alone in an apartment in Munich, anonymous even to the local officials.

He wasn’t listed in the phone book, had never held a job or paid taxes and hadn’t registered with the state-run health system. His only companions appeared to be more than 1,000 artworks that he kept in his darkened apartment.

The remarkable collection, with an estimated value of more than $1 billion, came to light only last year, when it was seized by German authorities who had questioned Mr. Gurlitt on suspicion of tax evasion. The discovery brought to light a decades-old mystery involving Mr. Gurlitt’s father, a German art dealer, and the Nazi looting of artworks from Jewish owners during World War II.

Dubbed “the Nazi treasure hoarder” by the German media, Mr. Gurlitt seemed overwhelmed by the attention and tried in vain to deflect attention from himself.

“I only wanted to live with pictures, in peace and calm,” he wrote in a Web posting this year.

Mr. Gurlitt died May 6 at his Munich apartment. He was 81 and had been recuperating from recent heart surgery. His death was announced on a Web site maintained by a group of his advisers.

Mr. Gurlitt seemed to pass through life unnoticed, almost a phantom. Although he had lived in a 1,076-square-foot apartment in a desirable neighborhood in Munich for more than 50 years, he was officially a resident of Salzburg, Austria. Officials found an additional cache of artworks in a neglected house he owned in Salzburg.

He had no close friends and, since the death of his sister in 2012, had no immediate family. There were no tax records, bank accounts, official health records or even a telephone number in his name.

His only income appeared to come from selling the occasional work of art. In Germany, the sale of artworks by private citizens is not subject to income tax.

Mr. Gurlitt’s sudden emergence in the public eye coincided with a resurgence of interest in the theft of paintings and other cultural artifacts during the Nazi era. “The Monuments Men,” a film about a U.S. military unit of art detectives, directed by and starring George Clooney, was released in February.

The real-life Monuments Men were well acquainted with Mr. Gurlitt’s father, Hildebrand Gurlitt. For about two years after the end of World War II, the elder Gurlitt was held under house arrest at a German castle.

When questioned by U.S. forces in June 1945, Gurlitt seemed “extremely nervous,” according the Monuments Men’s official report. The report described the elder Gurlitt as “an art collector from Hamburg with connections within high-level Nazi circles. He acted on behalf of other Nazi officials and made many trips to France, from where he brought home art collections. There is reason to believe that these private art collections consist of looted art from other countries.”

The elder Gurlitt had been a museum curator who had been dismissed from his job in 1933, in part because he had a Jewish grandmother. According to investigations by Vanity Fair magazine and the German publication Der Spiegel, he later gained the confidence of Joseph Goebbels, the Nazi chief of propaganda, and received a generous allowance to purchase art for a museum the Nazis planned to construct in Linz, Austria. It was never built.

Gurlitt was one of only four German dealers authorized by the Nazis to trade in “degenerate” art, meaning virtually any kind of modern art that Adolf Hitler — a recreational painter of landscapes — decreed to be morally objectionable.

By the late 1940s, the elder Gurlitt had been freed from detention and had settled in Dusseldorf, where he resumed his career as an art dealer and curator. He went about assembling the collection he had amassed during the war, reclaiming paintings he had stashed in a mill and other hiding places.

When he died in a car crash in 1956, the collection passed to his wife, Helene.

Cornelius Gurlitt was 23 at the time. He was born Rolf Nikolaus Cornelius Gurlitt in Hamburg on Dec. 28, 1932. Members of his family had been musicians, artists and scholars for generations. He studied art history at the University of Cologne, then lived in seclusion in Salzburg for a number of years.

After his mother died in the late 1960s, Mr. Gurlitt took possession of the collection and continued a life of monastic silence. When German authorities found him returning from Switzerland in 2010 with 9,000 euros in crisp new bills, they grew suspicious.

Inside his 1,076-square-foot apartment, they found a reported 1,406 works of art, including paintings by Picasso, Matisse, Renoir, Delacroix, Daumier, Courbet and Toulouse-Lautrec. At least 380 paintings were Nazi-designated “degenerate art,” and as many as 590 were of questionable origin.

In the only interview he gave in his life, Mr. Gurlitt told Der Spiegel in November, “I never had anything to do with acquiring the pictures, only with saving them.”

Even if the art had been stolen, under German law the works would not have to be returned to their original owners because the statute of limitations had expired. Ronald Lauder, president of the World Jewish Congress, said last year that the German law was the “principal obstacle to the recovery of Holocaust-looted art.”

Among the few facts Mr. Gurlitt revealed about himself were that he had not watched television since 1963 or seen a movie since 1967. When his paintings were taken from his apartment, he said it was a greater loss than the deaths of his parents and sister.

The paintings are now in a warehouse outside Munich, awaiting an uncertain fate.

“The saddest part of this whole story is this man’s life,” Munich art dealer Konrad O. Bernheimer told the New York Times in November. “He was locked up in the dark with all these wonderful paintings. He is a man in the shadows, a ghost who never came out.”