Police have stepped up security at airports in Germany amid concerns that extremists may have tried to scope out operations at Stuttgart airport.
German media reported on Thursday that security officials are investigating four people after they were spotted staking out the airport.
In a tweet, federal police said flights should not be affected by the "robust" police presence at Stuttgart airport.
Additional officers were also deployed to smaller airports in Baden-Wuerttemberg state - Friedrichshafen, Karlsruhe/Baden-Baden and Mannheim - as a precaution.
Public broadcaster SWR reported that two of the suspects, a father and son, were spotted taking pictures at France's Charles de Gaulle airport near Paris last week.
Last week, a gunman killed five people in an attack on a Christmas market in the French city of Strasbourg, just across the border from Germany.
The suspected gunman, 29-year-old Cherif Chekatt, died in a shootout with police two days after the attack on December 11.
Chekatt had been on a French intelligence watch list for radicalism and was convicted 27 times for criminal offences in France, Germany and Switzerland.
Separately, German authorities said they arrested two men and a woman in southwestern Germany on Wednesday on suspicion of illegal firearms ownership and preparing an attack.
Karlsruhe prosecutors said police seized a fully automatic weapon and ammunition in the raid, but that there was no indication of a link to the security alert at the nearby airports.
Additional reporting by Associated Press.
