MONTERREY, Mexico (AP)  Gunmen believed to be linked to drug cartels killed four police officers and a doctor in apparently coordinated attacks in and around the northern Mexican city of Monterrey , authorities said Thursday. Three officers were wounded in Wednesday's attacks, said Jorge Domene, a spokesman for the security council in northern Nuevo Leon state, where Monterrey is located. In one of the attacks, gunmen opened fire on a Monterrey police station, killing a medical doctor who was administering tests to employees at the station and wounding three officers, Domene said. In two additional, separate attacks carried out within minutes of each other in the Monterrey suburb of Guadalupe, gunmen killed two police officers, a man and a woman. "Yesterday's events clearly represent acts by organized crime trying to intimidate or reverse the actions that authorities have taken ... to counter the violence that has been unleashed in our state," Domene told a news conference. Monterrey, Mexico's third-largest city, has been rocked by drug-cartel turf battles. Six people were found dead Thursday in the southern state of Guerrero, another disputed drug trafficking hot spot, according to a state police statement. Two of the men were found in the resort city of Acapulco, one of them buried in a clandestine grave. He had been blindfolded and his hands and feet were bound. Also Thursday, the navy said in a statement that it had captured four suspects linked to drug-cartel activities, including two females under the age of 18. Marines detained the four Tuesday in the city of Matamoros, across the border from Brownsville, Texas, in two vehicles that had been reported stolen. Four assault rifles were found in the vehicles, and one of the male suspects claimed to be a hit man for the Gulf drug cartel. Mexican authorities have arrested astonishingly young suspects accused of working for cartels in recent months. Early this month, soldiers and police detained an American-born 14-year-old boy who allegedly worked as a drug-cartel enforcer. The boy told reporters he helped a drug gang behead four people. Under Mexican law, minors cannot be tried as adults and are handled under a separate juvenile-offenders system. The military, meanwhile, announced it seized a ton of marijuana per day in Baja California state this year, a record daily average. On Oct. 18, soldiers and police confiscated 148 tons of marijuana in Tijuana, Mexico's biggest pot haul on record. Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more