Investigators said on Friday there was an explosive device aboard a car that blew up outside police headquarters in the border state of Tamaulipas, the second car-bomb attack against law enforcement in less than a month.

Authorities are still working to determine what explosives were used, who planted the bomb and how it was detonated, according to a state police spokesman who was not authorized to be quoted by name.

No one was hurt by Thursday’s blast in a parking lot outside a police station near Ciudad Victoria, the state capital, but two police vehicles were damaged.

Interior Secretary Francisco Blake called the blast an “expression of crime” that seeks to create fear among the population.

“Criminals without scruples are using methods that seek to intimidate and, of course, terrorize the people, that try to create fear and inhibit us,” Blake said.

On July 15, a car bomb killed a policeman, a doctor and a kidnap victim used as a decoy in the border city of Ciudad Juarez, across from El Paso, Texas. A drug gang has been blamed for that attack.

Also on Friday, authorities in the western state of Sinaloa announced the discovery of nine bodies in a clandestine grave believed to be linked to drug gangs.

The bodies were found in a pit in the coastal city of Mazatlan, said Martin Gastelum, a spokesman for the Sinaloa state prosecutors’ office. An anonymous tip led police to the site.

Four of the bodies were missing their heads, and the victims appear to have been killed anywhere from a month to a year ago.

Meanwhile, the Mexican navy reported that four suspected gunmen for the Zetas drug gang were killed in a clash with marines in Tamaulipas on Thursday.

Nine assault rifles and three grenades were seized at the scene, the navy said.

And the federal Public Safety Department said in a report on Friday that a total of 2,076 federal, state and local police officers have been killed in attacks or executions by drug gangs since December 2006, when President Felipe Calderon launched an offensive against the cartels.

That figure is part of the total of more than 28,000 killings in drug-related violence in that period.

The federal department has criticized the lack of training and coordination among municipal police forces, which make up about 39 per cent of all officers in Mexico.

It said 68 percent of municipal officers in Mexico have only a grade-school education, and almost two-thirds earn salaries of $315 per month or less.

The department has proposed bringing police forces under a more unified command.

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