Four police headquarters have been attacked in the Monterrey metropolitan area of northern Mexico. The assailants used guns and grenades. Three people are confirmed dead.

At least 16 people were wounded and three were killed after attacks on four separate Mexican police headquarters on Saturday.

Assailants used grenades and guns in the attacks, which took place in the northern city of Monterrey and other surrounding towns.

During the first attack, a bystander and two suspected gunmen were killed in the crossfire. Another 12 police and four civilians were left injured.

The attacks appeared to have been coordinated - four grenades exploded in the space of about an hour.

The State Police Center, in the city of Monterrey, was a target. The other attacks were against police headquarters in the towns of Montemorelos, Allende, and Guadalupe.

Monterrey, located about 140 miles from the U.S. border, is Mexico's most affluent city. It generates eight percent of Mexico's gross domestic product and has the country's highest per-capita income.

Regional officials say that surging drug violence is undermining economic growth in Monterrey.

There have been some 650 drug killings this year in and around Monterrey, more than in the past four years combined for the region.

More than 29,000 people have died in Mexico since President Felipe Calderon launched his campaign to crush drug cartels in late 2006.