TORREON: Eight human heads were found early Tuesday in four separate places near roadsides outside the northern Mexico city of Durango, the local prosecutor`s office said.

Police found the heads within the space of two and a half hours due to anonymous tip-offs, "but had not identified the victims, nor located the bodies," said a statement from the Durango state prosecutor`s office.

The victims were males thought to have been aged between 25 and 30, the statement said.

The tip-offs said that two human heads had been dumped on each of three roads out of Durango city.

Workers cleaning a traffic island found the remaining two on a road leading north to the industrial city of Gomez Palacio.

The attorney general on Sunday accused officials at a prison in Gomez Palacio of releasing inmates to carry out drug-related killings. Those included three massacres which left 35 dead this year in neighboring Torreon, in Coahuila state.

Gruesome suspected drug killings have plagued Coahuila and Durango in recent months.

Authorities blame the violence on fighting between the Sinaloa gang, headed by Mexico`s most wanted man Joaquin "El Chapo" Guzman, and the brutal Zetas gang of former elite soldiers.

Some 25,000 people have died in spiraling suspected drug violence since December 2006, when President Felipe Calderon launched a military crackdown on organized crime.

BDST: 1010 HRS, July 28, 2010