BERLIN (Reuters) - Secret files of Communist East Germany’s Stasi security police were sent to a film set for use as props, triggering an investigation into how such sensitive documents were obtained.

The authenticity of the files were revealed when 15 former political prisoners were being filmed for a docu-play called “Staats-Sicherheit” (State Security) by public broadcaster ZDF.

“It’s just unbelievable that something like this could happen,” CDU politician and former East German civil rights activist Vera Lengsfeld told Reuters. “This must be cleared up right away.”

One of the “prop” files was actually the genuine file of one of the actors.

The German government’s agency that looks after the Stasi archives said they had began a probe into how the files were obtained without authorization.

The Birthler agency said it had also immediately ordered the Babelsberg studio to restrict access to safeguard the files.

“We’re investigating how these documents got into public hands,” said agency spokesman Steffen Mayer. He said a prop firm delivered the documents to the studio and it was assumed the files were fakes to protect the identities of the victims.

Lengsfeld, who spent time in jail in East Germany as a political prisoner, was one of the 15 political prisoners the film centers around.

She said she did not think it was a coincidence real files surfaced on the film set. She believes former Stasi employees now work at the Birthler agency.

“There were even real Stasi files from people who weren’t in the production being casually tossed about,” said Lengsfeld, who called it “scandalous” the documents were freely available.

After German unification Lengsfeld discovered her husband had been a Stasi informant. She then divorced him.

Among the files that surfaced on the film set were documents about one of the film’s actors, Mario Roellig. Roellig was arrested by the Stasi after a failed attempt to flee East Germany in 1987.

“I was just stunned when I saw them,” Roellig told the Leipziger Volkszeitung newspaper.

Also found were genuine Stasi files about a famous East German Rock musician and an artist, Lengsfeld said.