Police killed 15 armed assailants in a fierce gunbattle just south of the Arizona border on Wednesday after tracking a group of gunmen who killed five policemen into the nearby hills.

About 40 assailants, whose tactics and weaponry resembled those of Mexico's powerful drug gangs, drove into the town of Cananea, 20 miles south of the U.S. border, in up to 15 vehicles and seized four policemen in two patrol cars, Sonora state police said in a statement.

The bullet-riddled bodies of the four, along with 50 spent cartridges, were found on the side of a road hours later, the statement said.

Sonora state Gov. Eduardo Bours said a total of five policeman were killed in the day's fighting, but did not specify how the fifth officer died.

Three other officers were seized elsewhere. Two were released earlier in the day, and one was freed during the later gunbattle. Three Cananea residents who had also been aducted were freed.

After the initial attack on Cananea, the gunmen fled and tried to hole up in mountainous terrain around the town of Arizpe, about 50 miles to the south.

Police followed the assailants there, engaged in a shootout and killed 15 of the assailants, Bours said. Police seized 15 assault rifles and eight pistols following the hours-long confrontation, the Sonora state government said in a press statement.

Bours declined to speculate on a motive for the killings, but Mexico has seen a wave of attacks on police, military and intelligence officials as the government battles drug trafficking gangs.

President Felipe Calderon has vowed to crack down on Mexico's powerful drug cartels, which are battling for lucrative smuggling routes north, sending 24,000 soldiers and police to violence-plagued states to go after everyone from cartel leaders to growers and dealers.

Meanwhile, in Hermosillo, the capital of Sonora state, unidentified assailants on Wednesday tossed a hand grenade from a passing car at the offices of the newspaper Cambio. The device only caused minor damage and no injuries. A similar attack on the newspaper occurred in April.

And authorities in the northern state of Coahuila reported that men disguised as Mexican federal agents had kidnapped the state's chief anti-kidnapping investigator.

Lucio Tello, a spokesman for the state attorney general's office, said Enrique Ruiz Arevalo, director for investigations of kidnapping and organized crime for the agency, has been missing since Monday.

The four kidnappers wore black uniforms with the insignia of the Federal Agency of Investigation, Mexico's equivalent of the FBI, when they grabbed Ruiz Arevalo and another agent in Torreon, 310 miles southwest of the U.S. border at Eagle Pass, Texas.

They released the other agent hours later. Tello said the attorney general's office knew of no motive for the abduction.