On Friday, US Ambassador Richard Grenell demanded an investigation into a German journalist who was caught making up key details in a series of reports for Der Spiegel newsmagazine.

Grenell said the revelations "are troubling to the US Embassy, particularly because several of these fake stories focused on US policies and certain segments of the American people."

He said he wrote to the editors of the respected newsweekly to call for an "independent and transparent investigation."

Read more: Der Spiegel admits star reporter falsified stories 'on a grand scale'

Grenell said it was clear that the US had been the victim of institutional bias at the magazine, saying the outlet encouraged anti-American reporting, particularly since Donald Trump was elected president.

Journalist Claas Relotius, 33, resigned after admitting to making up stories and inventing protagonists in more than a dozen articles in the magazine's print and online editions. Since then, other outlets the freelancer wrote for, such as Die Welt and Die Zeit, have been poring over their articles.

Opinion: Truth is the first casualty of Claas Relotius

Spiegel responds

On Friday, Der Spiegel published a letter in response to Grenell's requests.

"We apologize to all American citizens who have been insulted and denigrated by these reports. We are very sorry. That should never have happened," Der Spiegel's Dirk Kurbjuweit wrote.

"However, I would like to contradict you on one point. If we criticize the American president, it is not anti-Americanism, but criticism of the policy of the man in the White House. Anti-Americanism is deeply foreign to me and I am absolutely aware of what Germany owes to the United States: a great deal. At Spiegel, there is no institutional bias towards the USA.

"You suggest that in other cases, too, there has been erroneous reporting about your country. Please tell us about these cases and we will investigate them immediately," Kurbjuweit wrote.

One of the cases that particularly infuriated the Americans was a story in which Relotius visited a small American town, claiming that he hoped to find out why people there had voted for Trump. He fabricated major elements of the story including people, quotes and geographical details. In another story he wrote about a woman who watches executions in America. In that case the woman does exist, but they only met briefly and he invented large portions of the story. In another case he wrote about a civilian militia on the border with Mexico in which he made up quotes and details.

On Saturday Der Spiegel published a 23-page special report on how the award-winning reporter faked stories for years, calling the deceit the "worst thing that can happen to an editorial team." The magazine apologized for the mistake and promised to "do everything to boost our credibility again."

Grenell later shared what he saw as examples of anti-American bias at the magazine.

Although Grenell said Spiegel's anti-American bias picked up after Trump's election, the image he shared was from 2007 and featured magazine covers related to former President George W. Bush.

as/sms (dpa, AFP, AP)

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