BERLIN — A former senior European diplomat and two others are being investigated on suspicion of spying for China, according to European officials, in a case that is sure to compound concerns about surveillance as Chinese telecommunications firms invest in Europe.

Apartments and offices in Germany — in Berlin, Baden-Wuerttemberg and Bavaria — and in Brussels were searched Wednesday over “allegations of espionage by secret agents,” said Markus Schmitt, a spokesman for Germany’s federal prosecutor. No arrests were made, he said.

The Belgian prosecutor’s office declined to comment.

The investigation comes at a time of heightened European concern about expanding Chinese spy networks in Europe. European Union officials were reported to have received internal warnings about the proliferation of Russian and Chinese spies in the vicinity of its Brussels offices last year.

European countries have also been under pressure from the Trump administration not to use the services of Chinese tech giant Huawei as it rolls out its 5G networks on the continent. U.S. officials have said the Chinese government could tap into Huawei equipment installed overseas to spy on the West or disrupt infrastructure. The company has denied the allegations.

Two European officials, who spoke Thursday on the condition of anonymity because of the sensitivity of the matter, identified the subject of the investigation as Gerhard Sabathil. Another senior E.U. official confirmed key biographical details that match Sabathil’s career path.

During his long career, the former diplomat held senior posts in a number of E.U. institutions, including the European Commission and the European External Action Service, the bloc’s diplomatic service. He was appointed E.U. ambassador to South Korea in 2015.

The senior E.U. official said Sabathil was recalled in 2016 after his security clearance was revoked. The German Hungarian national left diplomacy in 2017 to join the European lobbying firm Eutop.

Attempts to reach Sabathil through Eutop and his LinkedIn page were unsuccessful on Thursday, and the firm did not respond to requests for comment. The Munich-based firm counts Deutsche Telekom and British American Tobacco among its clients.

The other two suspects work for a separate lobbying firm, according to German news reports.

Virginie Battu-Henriksson, a spokeswoman for the E.U. diplomatic service, said she was aware of the media reports but could not comment on an “ongoing investigation” by German authorities.

“Of course we are always ready to cooperate with national authorities,” Battu-Henriksson said. She said that no E.U. buildings had been raided and that German authorities had not been in contact regarding the investigation.

“Obviously China has stepped up its presence in Europe, there’s no doubt about that, in quality and quantity,” said Philippe Le Corre, a senior researcher on China at the Harvard Kennedy School and co-author of the 2016 book “China’s Offensive in Europe.”

However, an investigation involving a European diplomat is highly unusual, Le Corre said. “It’s never happened in Europe, not like this.”

The European diplomatic service has undertaken “protective measures” against hostile intelligence agencies, Battu-Henriksson said. Those include ensuring staff is properly briefed on how to deal with both “human” and cyber threats, she said.

Aries reported from Brussels and Mekhennet reported from Washington. Luisa Beck in Berlin contributed to this report.