The California governor, Gavin Newsom, has spoken emotionally about the latest US mass killing, after an employee gunned down nine people at a California rail yard and then killed himself as law enforcement rushed in.

“There’s a numbness I imagine some of us are feeling about this. Because there’s a sameness to this,” he said. “It begs the damn question of what the hell is going on in the United States of America?”

It was the 15th mass killing in the nation this year, all of them shootings that have claimed at least four lives each for a total of 86 deaths, according to a database compiled by the Associated Press, USA Today and Northeastern University.