The museum was set to reopen after a renovation when the forgeries were discovered by a local art historian, Eric Forcada, in September. A simple glimpse at images of the paintings that Mr. Forcada received by email made him wary of their authenticity, he said in a phone interview. After inspecting the artworks and confirming his findings with a panel of experts, he found that 82 of the 140 pieces in the collection were counterfeits.

Most of the fakes were acquired after 2013 and were not on exhibit because town officials were awaiting the creation of a second floor in the museum to display its full collection, Mr. Forcada said. Still, 10 to 15 forgeries hung on the museum’s walls for years, and Elne’s mayor, Yves Barniol, called the fraud a catastrophe for his municipality.

The town council filed a complaint in March, leading the police to begin an investigation, and the museum reopened on Friday with about 60 paintings by Terrus, all authentic. Yet local officials said it could take years to find out who was behind the fakes.

Beyond the blow to the museum’s and the town’s reputations, the case has illustrated the risks that European small towns face when seeking to enhance a heritage that can galvanize cultural life and attract tourists. The Terrus museum had neither a curator nor a team dedicated to acquisitions.

Yet after Mr. Barniol became mayor of Elne in 2014, and as the museum’s founder was purchasing an increasing number of canvases in recent years, the town started to spend dozens of thousands of euros to restore some of the paintings, install better lighting and regulate the building’s temperature so the works could be better preserved.