On a sunny morning in late February, Yves Bouvier, a Swiss art dealer, flew into Nice and drove 20 miles along the French Riviera to Monaco to meet his top client, Russian billionaire Dmitry Rybolovlev. Bouvier had come to work out the final payment for Mark Rothko’s No. 6 (Violet, Green and Red), which Rybolovlev had agreed to buy for €140 million back in August. Bouvier, 51, entered the lobby of the cream-colored, belle époque mansion where Rybolovlev’s penthouse apartment overlooks Monte Carlo’s yacht-filled marina.

Assuming business as usual, Bouvier approached a man he thought was one of Rybolovlev’s bodyguards.