A German Army lieutenant appears to have taken advantage of his country’s chaotic system of processing migrants in 2015 to pose as a Syrian asylum seeker with the intention of carrying out an attack, the authorities said on Thursday.

Prosecutors said the soldier, whose name was not released, was a 28-year-old German from the city of Offenbach, near Frankfurt, who was stationed with the army at the Illkirch base in France, near the German border.

He is suspected of planning a violent attack, weapons violations and fraud, prosecutors in Frankfurt said. They were not aware of any specific targets, said Nadja Niesen, a spokeswoman for the prosecutors.

The man first caught the attention of Austrian authorities in January, when he stashed a loaded pistol in a cleaning shaft in a restroom at Vienna International Airport. He was detained on Feb. 3 when he returned for the weapon, prosecutors said.

Investigators discovered that the suspect, using a false identity, registered in December 2015 with German officials in Giessen, about 40 miles north of Frankfurt, as a Syrian refugee. It was not clear what steps officials took to verify his claim, given that he appeared to speak no Arabic, Ms. Niesen said.

“Why that did not catch their attention, I am not able to say,” she said.

The man was processed as a refugee and assigned to an initial processing center in Zirndorf, in the southern state of Bavaria, where he submitted an application for asylum. “He did all of this parallel” to his duties as a soldier, Ms. Niesen said. “He appeared to live a double life.”

More than a million people poured across Germany’s borders from 2015 to 2016, straining the registration system and leaving it vulnerable to exploitation. Since January 2016, the suspect had been drawing the monthly financial stipend provided by the German government for refugees, prosecutors said.

“This knowledge, as well as anecdotal evidence indicating the soldier had a xenophobic background, led to the suspicion that the defendant had planned to carry out a violent criminal act, an attack, with the weapon hidden in Vienna airport,” prosecutors said.

About 90 officers searched the man’s home, as well as locations in France and Austria, prosecutors said.

Another suspect, identified only as a 24-year-old student from Offenbach, was also detained, they said. The student knew about the soldier’s ruse and might have helped him, officials said.

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