Almost as soon as a gunman fired shots into a crowd during Saturday’s massive marijuana smoke-out in Denver, police were on a hunt for what has become one of their most powerful and accessible investigative tools: video. The Denver Post says detectives spent the night poring over not just surveillance from the park’s nine overhead cameras but also countless amateur photos and videos captured on onlookers’ cellphones. The department the next day released images from a YouTube video, zeroing in on a man who police at the time said was the gunman’s accomplice. The investigation of the shooting — as well as that of the Boston Marathon bombings, and a series of Denver hit-and-runs — underscores the growing importance of video footage in helping to solve crimes. Cameras are also increasingly trained on police officers themselves, capturing several high-profile encounters in recent years. Video surveillance is “very, highly, incredibly important to us,” said Denver police Sgt. Mike Farr. “It gives us something to give the public and begin the search and then narrow the search.” Ninety-eight cameras look out upon the city, compared to just 13 the city had before the 2008 Democratic National Convention.