Attorney General Marisela Morales said the police were from San Fernando, Tamaulipas, where authorities have unearthed mass graves holding scores of bodies. The body count likely to rise as digging continues, officials warned Thursday.

The police officers allegedly shielded from law enforcement four people -- three men and a woman -- authorities believe were directly responsible for the killings.

"All are members of the criminal organization Los Zetas," Morales said of the four, who were not identified.

The victims apparently were passengers on public buses running routes through San Fernando to Reynosa and Matamoros, two Mexican cities on the border with the United States.

"The federal government reiterates its commitment to solve these lamentable and reprehensible homicides, and end the corruption of the police force which has made a pact with organized crime," Morales said in a statement.

AFP