A federal judge in Oregon has sentenced a 75-year-old man to a year and a day in prison after authorities said he faked works by well-known artists and sold the knockoffs to a dozen unsuspecting buyers.

Larry Ulvi of Portland was sentenced Tuesday after prosecutors said a search of his apartment turned up 160 paintings, many done in the style of Mark Tobey. The artist, who died in 1976, was part of the Northwest School art movement, and his abstract-expressionist works have been shown around the world.

Prosecutors say Ulvi pocketed thousands of dollars from gallery owners and created uncertainty about the authenticity of Tobey paintings and drawings that remain in the marketplace or in the homes of art lovers.

Ulvi pleaded guilty to mail fraud last summer as part of a plea deal. Despite that, prosecutors and an FBI special agent said he has been unwilling to lead them to other fakes.

“The problem with fakes is that they take a long time to discover,” said Robert Koch, a Portland gallery owner who provided expert testimony. “One person may discover a fake but the tip of the iceberg is still buried under the water.”

Assistant U.S. Attorney Scott Asphaug wrote in his sentencing memorandum that Ulvi used his “skills as an artist and his mendacity as a salesman” to pull off a scheme that began more than decades ago.

According to court documents, Ulvi approached a California gallery owner in May 2013 and offered to sell work supposedly done by Tobey. The gallery owner paid $9,000 for three paintings, much less than what original Tobey works would fetch.

The gallery owner soon bought another half-dozen forgeries. When Ulvi offered three more paintings, the owner became suspicious that one man could own so many Tobey works.

The owner contacted Heiner Hachmeister, an expert on Tobey’s art. Hachmeister advised that the paintings were knockoffs. The expert had said the same thing to a Seattle dealer solicited by Ulvi in 2011 and 2012.

Besides the forgeries, the investigators who searched Ulvi’s northwest Portland apartment found a sheet of paper that had been used to practice the signatures of Tobey and another Northwest School artist, Kenneth Callahan.

Public defender Alison Clark asked U.S. District Court Judge Robert Jones to place Ulvi on probation, citing his age and health problems. She said her client never got rich off the scheme, and lives on fast food.

But the judge said Ulvi was a fraud for a “long, long time” and must be punished.

Jones said Ulvi is gifted, and will recommend he be sent to the federal prison in Sheridan, Oregon, which has “a wonderful art department.” The judge even said he’d visit.

“You can do good,” he said, “as long as you’re not making any fakes.”