Updated

Assailants shot dead four police officers in the western Mexican city of Aguascalientes on Thursday, the latest in a wave of slayings of law enforcement officers across Mexico.

The city police officers had arrived at the scene of a car crash when they encountered several men unloading firearms out of a vehicle and tried to arrest them, Aguascalientes state attorney general Xavier Gonzalez told a news conference.

A second vehicle arrived at the scene with about eight armed men, who fired on the officers, Gonzalez said. Three died immediately and the fourth died later in a hospital, he said.

Last week, more than a dozen armed men killed five agents and two secretaries in simultaneous attacks on two offices of the state attorney general in Acapulco.

Since Jan. 1, assailants have also killed six state and municipal police officers around the northern industrial city of Monterrey.

Investigators say the attacks on police are related to drug cartels that make billions of dollars smuggling narcotics to the United States.

President Felipe Calderon has sent more than 24,000 police and soldiers to areas ravaged by killings and promised there will be "no truce or quarter" against crime gangs whom he called the "enemies of Mexico. However, his offensive has not stopped the violence against police officers.