He initially wanted to work around the U.S.-Mexico border. But that turned out to be too dangerous, so he started looking for other subjects.
It was the beginning of 2011, which was a particularly violent time in Monterrey. Two drug cartels were fighting, King said, and young men from mostly poor communities were getting caught in the violence.
"A lot of the photography that I was seeing was just photography of dead bodies," King said. "I didn't feel those pictures were really telling me anything extra apart from somebody being brutally killed, and once you've seen a couple of those pictures you understand what's happening."
King wanted to go deeper.
"I wanted to understand why these young men were so vulnerable to this violence," he said. "What was happening in their world? Who were they? What was their reality?"
King spent more than two and a half years in Monterrey, exploring that reality. He concentrated a lot on portraits and trying to bring individuals across as best he could through his photographs.
The result is a series of intimate images that show tough-looking young men in quiet moments.
"It's important to show that they're just, in my opinion, vulnerable young men on the boundary between adolescence and adulthood and trying to work out what they're supposed to be doing in life," King said. "And at that point they've got this thing going on around them, just this crazy amount of violence, and people like them are getting pulled in."
He described a photograph he made of one such man, Samuel.
In the image -- No. 7 in the gallery above -- the young man is shirtless, showing tattoos on his chest. One of the tattoos is a picture of his murdered brother. King took that photograph, too.
"I took the picture of (Samuel's brother), and then a couple of days later, I gave him the print and then he was killed," King said. "And then we went to the funeral and the actual photograph was on the coffin.
"I thought, wow, this picture has taken on a completely new life, one that I would have never dreamt of, and one that's completely -- it's got nothing to do with me anymore."
The durability of the photograph surprised King, as did the steadiness of the violence around him.
"It's just never-ending," he said. "Even though the cartels come and go, the sort of dynamic of street gangs and drugs and mayhem and getting into fights, it's just non-stop.
"It was a surprise, and it wasn't a nice surprise to realize. I just didn't have an answer to how that could be stopped, how it could end, that cycle."