Counter-terrorism experts say they are highly concerned by the specificity of Harrach's message: "One would assume that they would not want to be shown to be bluffing and so, may actually have something planned," said Clarke.
Police in Stuttgart arrested a 25-year-old Turkish man Thursday for re-launching Harrach's on the Internet. The suspect had been under surveillance for previously supporting Islamist causes, and "has not created the video himself and it is not yet clear where he obtained the video from," police spokesman Stefan Kalbach said.
Chancellor Angela Merkel said security authorities were prepared to protect the country. The US State Department issued a Travel Alert on Wednesday cautioning Americans to be careful in coming weeks in Germany.
"Americans are advised to monitor news reports and consider the level of security present when visiting public places or choosing hotels, restaurants and entertainment and recreation venues" in Germany, the alert said. It expires on November 11.