Security forces searching mass graves in the northern Mexican state of Durango have unearthed eight more bodies, authorities said Wednesday, bringing the total to 188 and making it the largest discovery yet of corpses secretly buried in regions plagued by drug-gang fighting.

The toll surpasses the 183 bodies found in pits last month in Tamaulipas, a state bordering Texas.

Soldiers recovered the latest eight bodies Tuesday, and digging continued at five graves discovered last month in the state capital, Durango city, state Public Safety Department spokesman Fernando Rios told The Associated Press.

Durango authorities say some of the victims have been dead for up to four years while others were buried as recently as three months ago.

Although Durango investigators have refused to speculate on a motive for the killings, authorities have blamed drug cartels for other such mass graves discovered over the past year in Mexico.

Durango, a large mountainous state where authorities suspect some of the most-wanted drug kingpins may be hiding, has been a battleground between the Sinaloa, Zetas and Beltran Leyva cartels.

The state's homicide rate has more than doubled over the past two years. At least 1,025 homicides were reported in the state in 2010, compared to 930 in 2009 and 430 in 2008, according to government figures.

Families of people who have disappeared in Durango have come forward to ask whether their relatives may have been buried in the mass graves, according to deputy state Public Safety Secretary Juan Rosales. But he said the painstaking identification process has overwhelmed the state government, prompting them to seek help from federal authorities.

In Tamaulipas, authorities have said many of the people buried in the graves may have been recently pulled off passenger buses by the Zetas, which control much of the state and have been kidnapping Mexicans and foreign migrants to demand extortion money or forcibly recruit them.