IJUANA, Mexico (AP) — Authorities found the decapitated bodies of four men Tuesday in Tijuana, while a grenade attack on a police station in western Mexico wounded a pregnant woman and her 3-year-old daughter.

Three of the decapitated bodies in Tijuana were inside a house, the Baja California state prosecutor's office said in a statement. An hour later, police found a fourth decapitated body inside a pickup truck.

The office said the heads were left near the bodies along with messages. It didn't say what the notes said.

Tijuana has been wracked by an upsurge in drug cartel violence in recent years. Prosecutors say 583 people have been killed so far this year in the city.

Meanwhile, assailants hurled three grenades at a police station in the western city of Morelia, injuring the pregnant woman, her daughter and a police officer, Michoacan state Public Safety Secretary Minerva Bautista said.

The woman, who is seven months pregnant, and the toddler were hospitalized in stable condition, while the officer was sent home with minor injuries.

The attack came less than an hour after assailants threw two grenades into a busy road outside a Morelia high school just 300 meters (yards) from the Michoacan state government headquarters. The midday attack caused chaos and panic but no injuries.

Michoacan, a Pacific coast state, has become a major battleground between the Gulf and La Familia drug cartels, with innocent bystanders increasingly getting caught in the crossfire.

Mexico's drug gang violence has surged since President Felipe Calderon deployed tens of thousands of troops and federal police across the country in late 2006 to crack down on brutal cartels. More than 14,000 people have since died in violence tied to the drug trade