Transcript for Gunman on Train Goes Unnoticed Before Killing
And staying in san francisco, there's a shocking crime that happened on a crowded train there. Police say this man had been waving and gesturing with a gun on the train, just minutes before shooting another passenger. And apparently, no one noticed because they were glued to their smartphones and tablets. Abc's lindsay davis has that story. Reporter: Police say it was a senseless crime that could have been avoided. My sister is the closest thing I have in this world. Reporter:20-year-old justin valdez, seen in this youtube video honoring his family, was shot in the back of the head, as he exited a san francisco train, by a man who flashed a gun several times in front of distracted, gadget-absorbed commuters. Surveillance video of the incident, which happened late last month hasn't been released, with the exception of these images, the man police say shot valdez. The rest of the footage shows him pulling out the gun several times, right in front of roughly a dozen commuters, including valdez, who had his back to him. But no one on the train noticed. You can see people sitting or standing around him. And they're working with their tablets or smartphones, completely distracted. Reporter: Police say it was completely unprovoked, that the two men didn't even know each other. I miss him. I miss him all day. He was too young to die. Reporter: He is expected to appear in court next month. He has pleaded not guilty. For "good morning america," linsey davis, abc news, new york.
This transcript has been automatically generated and may not be 100% accurate.