Hong Kong (CNN Business) A top European news magazine has fired one of its star journalists after discovering that he had fabricated facts and sources in more than a dozen articles over a seven-year period.

"Claas Relotius, a reporter and editor, falsified his articles on a grand scale and even invented characters, deceiving both readers and his colleagues," Germany's Der Spiegel said in an article published online Wednesday.

The startling revelation is a heavy blow to Der Spiegel, a 71-year-old publication that's renowned for its quality journalism and read by hundreds of thousands of people in print and by millions online.

"I'm so angry, horrified, shocked, stunned," the magazine's deputy foreign editor Mathieu von Rohr tweeted. "Claas Relotius faked, he cheated all of us."

After a colleague working with Relotius on a story in the United States flagged suspicions about his reporting, Der Spiegel says it carried out an internal investigation. Relotius confessed last week that he invented entire passages for that article, and also falsified information in other stories, according to the magazine.

Relotius, who resigned Monday at the magazine's request, said he was sick and needed help, a Der Spiegel spokesman told CNN.

CNN hasn't been able to reach Relotius for comment.

Award-winning stories

His work had received awards from CNN, but a CNN spokesperson said, "Relotius was not associated with CNN, he never worked for the company and never had anything published on CNN platforms."

Relotius first joined Der Spiegel in 2011 as a freelancer, and was hired as an editor a year and half ago.

Der Spiegel said the 33-year-old admitted to partial fabrications in at least 14 of nearly 60 articles he wrote for its magazine and website. That included making up dialogue and quotes and creating composite characters. The magazine warned that the number could be higher, and that other news organizations could be affected.

Relotius has published stories in other leading German-language publications, including Süddeutsche Zeitung Magazin, the Financial Times Deutschland, Die Welt and Frankfurter Allgemeine Sonntagszeitung, according to Der Spiegel.

He also penned a feature for Swiss magazine Reportagen, for which he received two CNN Journalist Awards in 2014, including Journalist of the Year. The CNN Journalist Awards were for German-language journalism and decided by an independent jury. The last awards were given in 2015.

A spokesman for CNN said Thursday that the 2014 awards jury held a meeting after Der Spiegel went public with its investigation, and voted unanimously to strip Relotius of both awards.

Forbes identified Relotius as a top reporter last year, including him on a "30 Under 30" list for European media

Several major features Relotius wrote for Der Spiegel that were also nominated for or won journalism awards are now under scrutiny, according to the magazine.

Among them, "The Last Witness," about an American who allegedly travels to an execution as a witness, "Lion Children," about two Iraqi children who have been kidnapped and reeducated by the Islamic State, and "Number 440," a feature about alleged prisoners at the US detention facility in Guantanamo Bay, Cuba.

Investigation continues

Der Spiegel said it is still combing through the allegations against Relotius and will set up a commission made up of people from both inside and outside the company to investigate his work.

For now, Relotius' stories will be left unaltered online, but with a notification telling readers that his reporting is under suspicion of extensive forgery and manipulation.

Frankfurter Allgemeine Sonntagszeitung said Thursday that it was reviewing the stories by Relotius it had published. It said that no evidence of forgery had yet been found.

Der Spiegel is the latest prominent news organization to be rocked by revelations of a high-flying reporter falsifying information.

In 2003, The New York Times discovered that one of its rising stars, Jayson Blair , had invented parts of his stories and stolen material from other news outlets. The scandal resulted in the resignations of Blair and the newspaper's top two editors.