SAN FRANCISCO — A 20-year-old man was shot to death while playing "Pokémon Go" at a tourist attraction along San Francisco's waterfront, authorities and a family friend said on Sunday. Calvin Riley was shot Saturday night by an unknown assailant at Aquatic Park near Ghirardelli Square, the US Park Police said. A family friend told KGO-TV that Riley and a friend were playing the hit mobile game when he was shot. "From what we know there was no confrontation," John Kirby said. "There was nothing said back and forth. It was just senseless, just came up and shot in the back and ran away for nothing." Kirby said Riley and his friend had noticed someone watching them from the top of a hill that overlooks the park, but it was dark and they were mostly looking at their cellphones. National Park Service spokesman Lynn Cullivan said investigators believe there are witnesses to the attack because it happened in the bustling Fisherman's Wharf area. They are seeking tips. Sgt. Robert Jansing, a detective with the US Park Police, said the gunman did not take any of Riley's belongings or try to rob him. It was unclear whether the attack had anything to do with "Pokémon Go," which has become a national sensation this summer as legions of fans visit real-world landmarks to find and trap cartoon creatures. But the game's popularity has created unintended consequences in everyday life. Property owners complain of players trespassing or damaging their grounds, and scores of players have been injured or robbed as they stare at their phones in search of digital monsters. Law-enforcement officials have warned about the game's possible dangers, saying players can become engrossed in the game and tune out their surroundings.