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Mexican security forces are often accused of abuses Reuters

An army lieutenant and three soldiers have been arrested in Mexico over the summary killing of seven people in the latest episode of alleged abuses to tarnish security forces' reputation in the country.

Investigators from Mexico's National Human Rights Commission said there is evidence the four were responsible for the deaths of seven labour workers whose bodies were found with a bullet wound to the head in a common grave in the central state of Zacatecas.

The victims, five men and two women, disappeared last week from the town of Calera, with local media reporting they had been arrested during an army raid for drugs and weapons. Family members maintained they were simple farm workers employed in the area.

Earlier this week the military announced an internal investigation had found indications that members of its 97th infantry battalion were involved.

Authorities later confirmed that four military personnel, including lieutenant Martín Pérez Reséndiz, were under arrest over the case.

In a similar episode last year, three soldiers were charged with murder over the execution of 22 suspected gang members in the rural town of Tlatlaya, south-west of Mexico City.

The development in Zacatecas came just days after a 12-year-old boy was shot dead as armed forces opened fire on demonstrators protesting the arrest of a self-defence group leader in the western state of Michoacan. Another four people were injured in the incident in Aquila municipality.

Authorities said the shooting was initiated by unknown gunmen, while protesters said federal forces opened fire as they tried to block a highway.