Swiss police believe that two Russian spies posed as plumbers in order to bug the Davos World Economic Forum and keep tabs on world leaders, the Swiss Tages-Anzeiger newspaper reported Monday. Police in August took custody of the two men, who aroused suspicion due to their unusually long stay at the luxury resort.

The two Russians had diplomatic passports, but were not registered as diplomats in Switzerland. The police believe the pair were doing “preparatory work” for spying on the World Economic Forum, with the Russians possibly involved in a wiretapping or hacking operation.

“We did not receive any information about this from Swiss authorities. There is no evidence of spying,” Russian embassy spokesman Stanislov Smirnov said in response to the allegations.

Espionage allegations have caused friction in the political relations between Russia and Switzerland in the past. In a 2018 report, a Swiss government agency alleged that one in four Russian diplomats in the country was a spy.

The World Economic Forum in Davos-Klosters, Switzerland, takes place annually and “engages the foremost political, business, cultural and other leaders of society to shape global, regional and industry agendas,” according to the conference’s website.

Some of the participants in the conference include President Donald Trump, President of the European Commission Ursula von der Leyen and climate activist Greta Thunberg.