In the darkness, the team suits up quietly, putting on their helmets and tactical gear. Federal agents lug battering rams, bolt cutters and heavy weaponry by foot up a hill on a residential California street that's softly aglow from street lamps. Then the agents turn onto the walkway of their target's home.

Drug cartel gunmen ambushed three SUVs along a dirt road, slaughtering six children and three women — all U.S. citizens living in northern Mexico — in a grisly attack that left one vehicle a burned-out, bullet-riddled hulk, authorities said Tuesday.