Six police officers were shot to death in an apparent ambush in a rural part of western Mexico that has been plagued by drug violence, prosecutors said Tuesday.

The bullet-riddled bodies of an investigative officer, a group commander and four officers were found after a caller reported gunshots late Monday near Aguililla, a mountain town about 200 kilometers (125 miles) southwest of the Michoacan state's capital of Morelia.

The area is frequently used by drug traffickers, and dozens of suspected traffickers and Michoacan police officers have been killed in recent months amid gang turf battles.

A spokeswoman for the Michoacan state attorney general's office confirmed the killings, and said the officers had been on a routine patrol when they were attacked by unidentified assailants.

Local media reported the officers had been shot multiple times, and that a note was found near the bodies that read "Greetings, Luis Valencia, the Family sends it greetings."

Similar notes have been encountered next to many of the 17 severed human heads found in Michoacan this year.

"The Family" is believed to be a reference to a group linked to Mexico's Gulf drug cartel. "Luis Valencia" was an apparent reference to a drug trafficker by that name who operates in Michoacan.

There have been more than 420 homicides in the state this year, including 19 police chiefs and commanders. Juan Antonio Magana, the state's attorney general, has said well over half the killings were drug-related.

The battles involve leadership struggles within and among gangs, after authorities captured some of their top leaders.