A Mexican judge has ordered 16 police officers to remain detained for 40 days over their alleged involvement with the Zetas drug gang in a region where a series of mass graves were found.
The police, who were transferred to Mexico City on Monday, are under investigation for probable involvement in organised crime, kidnapping and murder, the federal attorney general's office said in a statement.
The officers were suspected of collaborating with Zetas gang members accused in the killings of at least 145 people found in mass graves since early this month in San Fernando, Tamaulipas state, near the US border.
Mexican troops on Saturday arrested a suspected Zetas leader, Martin Omar Estrada Luna, alias 'El Kilo,' believed to be linked to the mass graves.
Authorities also blame the Zetas for the massacre of 72 migrants in Tamaulipas last August.
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A new police chief, Rafael Lomeli, took over in Tamaulipas on Monday after his predecessor resigned.
The state has suffered an explosion of violence for more than a year blamed on battles between the Zetas, a gang formed in the 1990s by ex-elite soldiers, and their former bosses, the powerful Gulf cartel.
The Inter-American Commission for Human Rights on Monday called on the Mexican government to investigate the latest killings and urgently take measures to protect migrants crossing its territory.
Elsewhere, Mexican officials on Monday announced the arrests of 28 suspected Zetas in central Hidalgo state in connection with a car bomb, in which a police officer died, and six other killings in the area last year.
More than 34,600 people have died in an explosion of drug violence since 2006, according to official figures, amid a widespread military crackdown on organised crime.
