Mr. Relotius, 33, had been writing for the magazine since 2011, and he admitted to making up parts of at least 14 articles in Der Spiegel, the magazine said. That figure could rise, it added. Mr. Relotius wrote almost 60 articles for Der Spiegel, and as a freelancer wrote for a variety of German-language newspapers and magazines.

He did not immediately respond to a request for comment.

In one article, about Fergus Falls, Minn., Mr. Relotius told of a coal plant employee named Neil Becker (who does not exist), related an anecdote about a restaurant employee (to whom he gave a fictional illness and a misnamed son) and described the view of a power plant from a cafe (whose few windows provide no such view), according to Michele Anderson and Jake Krohn, two residents of the town who, baffled by the claims, spent a year researching them.

“It was such a weird story, it was so completely different than the town and the people that we knew,” Mr. Krohn said in an interview. “We thought we were at first completely missing something.”

Mr. Krohn and Ms. Anderson initially tried to contact Der Spiegel on Twitter, they said, but eventually decided to investigate the article’s claims on their own. Mr. Krohn, a technology consultant, and Ms. Anderson, who works in a community arts program, published their work on Medium after learning Mr. Relotius had been fired.

In one instance, Ms. Anderson said, Mr. Relotius falsely attached the words “Home of Damn Good Folks” to a sign that merely says “Welcome to Fergus Falls.”