Heavily armed police were patrolling late Thursday in several airports, including Stuttgart, Karlsruhe/Baden-Baden, Friedrichshafen, and Mannheim, according to a federal police spokesman.

The reinforced presence of police will remain in place until further notice, the spokesman added.

At the same time there has been no disruption to flights, a spokeswoman for 
the airports said.

Some German media have speculated as an explanation for the beefed up security the spotting of four men, including a radical Islamist, allegedly suspected of planning an attack, which police did not confirm.

Several days ago, the men were filmed by a surveillance camera at Stuttgart airport, the daily Tagesspiegel reported. They remained near the metal detectors without taking a flight and without baggage.

And Südwestrundfunk,  a media group in western Germany, reported that Moroccan authorities had informed their German counterparts that they had 

intercepted conversations referring to a possible attack at an airport near the German-French border.

Small airports have also reinforced security controls “as a precaution”, police at Reutlingen said.