Reuters/Grigory Dukor
The man in question was identified as "Ivan A.," a physicist who was appointed to the Dutch Eindhoven University of Technology in 2013, and visited the Max Planck Institute in Bonn, Germany, as a guest lecturer three times somewhere between 2009-2011.
The German intelligence agency's suspicions were aroused when they noticed the physicist meeting with a Bonn-based Russian diplomat once a month.
The diplomat, who Germany identified as a Russian foreign intelligence officer, according to Newsweek, reportedly gave money to Ivan A. in exchange for information during these meetings.
Ivan A. and his wife were later arrested in Düsseldorf Airport in 2014. He was released shortly after, but his photo and fingerprints were taken and a formal investigation was launched.
The Dutch Ministry of Foreign Affairs revoked Ivan A.'s visa when he returned to Eindhoven, and he has since returned to Russia.
The NL Times notes that Ivan A. continues to deny any involvement in espionage activities, maintaining that he received the money for renting a Moscow apartment to the diplomat's friends.