A former assistant to renowned American contemporary artist Jasper Johns pleaded guilty on Wednesday to selling nearly two dozen of Johns' works without his knowledge.

James Meyer, 52, pleaded guilty to interstate transportation of stolen property in Manhattan federal court before U.S. District Judge J. Paul Oetken, just over a year after his indictment.

Meyer worked as Johns' studio assistant for more than 25 years. Between 2006 and 2012, prosecutors said, Meyer stole 22 works of art that Johns had not yet completed from his Connecticut studio and brought them to a Manhattan art gallery for sale, claiming Johns had given the art to him as a gift.

In an effort to conceal the fraud, Meyer created fake notarized certifications that purported to document the gift and false inventory numbers for the artwork, which was eventually sold for a total of $6.5 million, prosecutors said. Meyer collected $3.4 million in proceeds, according to the indictment.

Meyer faces up to four years in prison when he is sentenced in December. He also agreed to forfeit nearly $4 million to the government.

The plea came eight months after a metal shop owner, Brian Ramnarine, pleaded guilty in the midst of his criminal trial to passing off a sculpture he created as a genuine Johns piece and attempting to sell it for $11 million. Johns himself testified at trial as a prosecution witness.

Ramnarine is scheduled to be sentenced next month on three counts of wire fraud, which also stemmed from his efforts to sell works he falsely claimed were made by Brazilian-born artist Saint Clair Cemin and American pop art sculptor Robert Indiana, creator of the famous sculpture "Love."

Johns, 84, is known for using images from American popular culture in his work, which includes painting, print-making and sculpture.