A 25-year-old is shot dead trying to sell marijuana the old-fashioned, illegal way. Two men from Texas set up a warehouse to grow more than they would ever need. And three people buying pot in a grocery store parking lot are robbed at gunpoint.

While no one expected the state’s first-in-the-nation recreational sales would eliminate the need for dangerous underground sales overnight, the violence has raised concerns among police, prosecutors and pot advocates that a black market for marijuana is alive and well in Colorado.