Eight severed human heads were found in the Mexican city of Durango, authorities said.
Police were directed to some of the heads by a series of anonymous telephone calls that began shortly before 7 am Tuesday, the Durango state attorney general's (AG) office said.
The eight heads were scattered among various roads on the edge of the city, according to a statement from the AG office, which said authorities had yet to find the corresponding bodies or identity the victims.
While some of the heads were found as a result of the phone calls, others were discovered by maintenance workers in city parks.
All of the victims were men between the ages of 25 and 30, the AG office said.
The gruesome discoveries in the state capital come two days after the arrest of the warden at the Cereso 2 prison in Gomez Palacio, Durango, on accusations she allowed inmates to leave the facility to carry out killings.
Margarita Rojas Rodriguez and several subordinates are charged in connection with more than 30 deaths, including the July 18 massacre of 17 people attending a birthday party at a rented hall in Torreon, Durango.
Rojas alleged permitted inmates to leave Cereso 2 prison at night to carry out killings with active help from some of the guards.
The inmates returned to their cells after the crimes.