German police step up security at airports in southwest Police stepped up security at airports in southwestern Germany on Thursday, amid concerns that extremists may have tried to scope out operations Stuttgart airport

BERLIN -- Police stepped up security at airports in southwestern Germany on Thursday, amid concerns that extremists may have tried to scope out operations at Stuttgart airport.

German news agency dpa and public broadcaster SWR reported that security officials are investigating four people, including at least one with possible ties to the Islamist scene.

In a tweet, federal police said flights shouldn't be affected by the "robust" police presence at Stuttgart airport. Additional officers were also deployed to other, smaller airports in Baden-Wuerttemberg state — Friedrichshafen, Karlsruhe/Baden-Baden and Mannheim — as a precaution.

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 Dec. 11 attack. Chekatt had been on a French intelligence watch list for radicalism and was convicted 27 times for criminal offenses 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.