Russian police detained a man they suspected of spying on the England training camp, local media has reported.
Citing sources in the Russian security agencies, the Fontanka publication said that the man had been spotted acting suspiciously on a roof approximately 180m away from the base on Tuesday.
He was detained by security officers and taken away to a local police station.
There, he was asked to delete panorama photos he had taken of the English base, and released without charge.
The man, an employee of the Rostelecom state phone company, said that he was on the roof as part of routine repair work. He had not intended to observe the England team preparations, he said.
The publication says the man was suspected of spying for the Croatians, but offers no evidence of any link.
Tensions between Moscow and Zagreb have soured over recent days, after a video showed Croatian defender Domagoj Vida performing a populist, pro-Ukrainian greeting, alongside assistant coach Ognjen Vukojevic, who was then fired.
In a video posted to Facebook and subsequently uploaded to YouTube, Vida shouted “Glory to Ukraine!”, while Vukojevic added: “This victory is for Dynamo [Kiev] and Ukraine.”
England play Croatia this evening, at the Luzhniki stadium, for a place in the final. It would be their first final since England’s only victory in the World Cup, in 1966.
