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 on Saturday night by an unknown assailant at Aquatic Park near Ghirardelli Square, the US Park Police said.A family friend told KGO-TV Riley and a friend were playing the hit mobile game when he was shot."From what we know there was no confrontation. There was nothing said back and forth. It was just senseless, just came up and shot in the back and ran away for nothing," John Kirby said.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 spokesperson 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.Sergeant 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's unclear whether the attack had anything to do with Pokémon Go, which has become a national sensation this season as legions of fans visit real-world landmarks in order 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 to their surroundings.