It's not just a war between the drug cartels and the Mexican government. It's a war within the Mexican government.
It's not declared, but it's real.
The men who killed the police chief, were probably the police. At a minimum, they were people working with corrupt police and with their blessing.
The same thing happened in Cancun several months ago. They killed a recently retired Army general who had a reputation for being incorruptible who was brought in by the mayor to police the narco-corrupt police. The police chief was arrested for planning the murder. This is going to get worse before it gets better.