In late May of 2008, a watercolor sketch of a woman in a yellow dress, with a red belt and blue shoes, arrived at the Oklahoma City Museum of Art. The initials L.V., in the lower right-hand corner, stood for Louis Valtat, an acquaintance of Henri Matisse. The sketch came with a copy of a page from an auction catalogue, as evidence of the piece’s provenance, and a letter from someone in Mississippi named Mark Landis. Landis asked that the work be accepted in memory of his father, Lieutenant Commander Arthur Landis, Jr., U.S.N. The museum already had a Valtat oil. To make room beside it, the curators took down a Renoir.

A month later, an older man, dressed in dark clothes and carrying a briefcase, appeared. He was frail, and he stooped; his ears stuck out; he was bald, with a high forehead and blue eyes; and he was very pale—Mark Landis. He had a feathery voice and talked so incessantly that his company was fatiguing. To Matthew Leininger, who oversaw the curatorial department, he seemed “weird and eccentric,” qualities characteristic of philanthropists, in his experience. “I took him for a typical unknown art collector,” Leininger told me.

In the briefcase, Landis had five works that he proposed to donate in honor of his father. He said that he was having heart surgery and wanted to disperse the pieces beforehand. He also suggested that he had more work to give. As a donor the museum hoped to cultivate, “he was treated like royalty,” according to Leininger. He stayed for two and a half days. The director took him to the gift shop and told him to choose whatever he liked, and he left with two bags, filled mostly with books. The chief curator drove him to the airport. A few hours later, Landis called to say that he had fallen asleep and missed his flight, and asked if someone could help him arrange a new one, so the curator went back to the airport. While Landis slept, someone had stolen his books.

A couple of months later, Leininger began gathering information on the Landis donations in order to provide worksheets to the museum’s trustees, a formality that is generally observed before an object is added to the permanent collection. The first piece he considered was a watercolor by Paul Signac, an early practitioner of Pointillism. The painting didn’t have a title, but there were two boats in it, so Leininger did a search for “Paul Signac, double boat,” then “Paul Signac, Mark Landis,” but nothing came up. When he searched images, though, the painting appeared, in a press release issued by the Savannah College of Art and Design, as a gift from Mark Landis. “I didn’t think anything about it, because artists often do the same subjects,” Leininger said. “Think of Monet and his cathedrals.”

The next piece Leininger looked at was an oil on panel from the nineteenth century, by the French artist Stanislas Lépine. It depicted horses and a cart in a field, where hay was being harvested, on the outskirts of a town. Leininger found it in a press release from the St. Louis University Museum of Art, given by Mark Landis.

One means of determining the age of a painting is to examine it under black light. Different pigments, each prominent in different periods, turn different colors. Parts of the Lépine glowed a bluish white, a possible indication of new paper. In addition, Leininger said, “it smelled of linseed oil,” which would long since have dissipated.

Leininger then considered a red chalk drawing of a reclining nude, attributed to the seventeenth century, in which, over what ought to have been a uniform surface, “all sorts of anomalies—light spots and dark spots—appeared.” The drawing was attached to a mat, and with a scalpel he cautiously separated one of the mat’s corners. “The edges are brown, and it’s brittle, and it’s probably going to break,” he said. “It looks very old—but when I peel the layers apart it’s stark white. Brand-new. The brown spots smell like stale coffee. I think, Did Landis know that what he’s owned all these years was fake?”

Leininger sent an e-mail to members of the American Alliance of Museums asking if anyone had received gifts from Landis. “In the first hour, I had about twenty people contact me,” he said. By the next day, he was able to determine that several museums held the same painting. “There’s an Alfred Jacob Miller that’s at six or seven institutions,” he said. “The Lépine is five places, including the Art Institute of Chicago and the Art Museum at the University of Kentucky, and there’s a Marie Laurencin self-portrait in five places. When I give my director this information, she’s, like, ‘Oh, my God, take down that Valtat now!’ ” They did, and they put back the Renoir.

Leininger is forty, tall and imposing, with a forceful voice. He has a master’s degree in fine art, as a printmaker, and he is a knowledgeable follower of Nascar, which his wife introduced him to while they were courting. After the Valtat came down, he began trying to find out whatever he could about Landis. It became a passion with him. He records the name of everyone he speaks to about Landis, and every donation he uncovers.

Eventually, Leininger learned that Landis has given fraudulent works to more than fifty museums in twenty states, some of them on multiple occasions. The earliest of Landis’s gifts that Leininger knows of, which was actually his third, was made in 1987, to the New Orleans Museum of Art. This was “Portrait of a Young Girl,” by Marie Laurencin, a French painter who died in 1956. Many museums never discovered the forgeries; some eventually did, often through Leininger’s efforts; and a few saw through them almost immediately, partly because Landis seemed so odd.

The more Leininger pursued Landis, the more elusive Landis became. In 2009, an official at the Louisiana State University Museum of Art told Leininger that the museum had been paid a visit by a peculiar man. A photograph of the man showed that he was clearly Landis, but he had said that he was Stephen Gardiner. In the name of his mother, Joan Green Gardiner, he had given a drawing of a woman lying on a chaise longue, done, he said, by Watteau in 1719.

In 2010, Landis, as Father Arthur Scott, a Jesuit priest, gave the Mint Museum, in Charlotte, North Carolina, a pastel by Everett Shinn, a member of the Ashcan School. Landis made the donation in memory of his mother, Helen Mitchell Scott. Father Scott drove a red Cadillac. When people asked what his duties were, he sometimes said that he travelled a lot and solved problems for various Jesuit institutions. He gave the Oglethorpe University Museum of Art, in Georgia, a drawing, by an unknown French artist, of a woman playing a lute, and to the University of Louisiana at Lafayette he gave an oil by the Kentucky artist Charles Courtney Curran.

In July of 2011, Landis, as Father James Brantley, gave an oil on copper by the sixteenth-century painter Hans von Aachen to Cabrini High School, in New Orleans. In February of 2012, as Mark Lanois, he gave “Christ on the Way to Calvary,” an oil on copper, by Paolo Landriani, an artist of the sixteenth and seventeenth centuries, to Loyola University New Orleans. For a long time, Leininger didn’t know where the drawings and paintings Landis donated came from, but not long ago he learned that Landis made them himself.