The American ambassador to Germany has accused one of that country’s leading newsmagazines of anti-American bias after a journalist admitted to fabricating details in at least 14 stories — including one about the kind of rural town that propelled President Donald Trump to victory in the 2016 presidential election.

In a Friday letter to the news magazine, Der Spiegel — and in a series of tweets over the weekend — the ambassador, Richard Grenell, said it “was clear we were targeted by institutional bias.”

“We are concerned these narratives are pushed by Spiegel’s senior leadership and that reporters are responding to what the leadership wants,” Grenell wrote in the letter, adding that he wanted an outside, independent investigation to determine how the magazine violated journalistic standards after repeatedly publishing the work of 33-year-old Claas Relotius.

In an open letter published Saturday, Der Spiegel’s deputy editor-in-chief Dirk Kurbjuweit apologized for the fabrications and acknowledged that the publication’s verification processes broke down. But he rejected Grenell’s characterization of bias, saying that criticism of President Donald Trump does not equal institutional bias.

“When we criticize the American president, this does not amount to anti-American bias — it is criticism of the policies of the man currently in office in the White House,” Kurbjuweit wrote. “Anti-Americanism is deeply alien to me and I am absolutely aware of what Germany has the U.S to thank for: a whole lot.”

Der Spiegel said last week that Relotius had committed journalistic fraud “on a grand scale” over many years.

German journalist Claas Relotius holds his trophy for the CNN Journalist Award 2014, which was stripped from him this month when he admitted to fabricating details in stories. Gert Krautbauer / EPA file

One of his stories with fabricated details was about an American woman who supposedly volunteered to watch death row executions. Another was about American vigilantes on the U.S.-Mexico border. A third was about Fergus Falls, Minnesota — a story that Der Spiegel said “painted a tendentious, malicious portrait of the small, rural town” and “leaned heavily on ugly, misleading prejudices.”

In a post on Medium, two Fergus Falls residents described what they called Relotius’ 10 “most absurd lies” — including his assertion that the town is obsessed with the film “American Sniper” and a description of the city administrator as a “gun-toting virgin” with a taste for 18th-century French philosophers.

Relotious’ fabrications were revealed after a colleague accused him of falsifying details in “Jaeger’s Border,” a story that Der Spiegel said contained similarities to a 2016 piece about the border published by Mother Jones.

CORRECTION (Dec. 24, 2018, 10:20 a.m. ET): An earlier version of this article misidentified a city that Claas Relotius wrote about. Fergus Falls is in Minnesota, not Idaho.