A roadside bomb exploded near a US Humvee in western Afghanistan on Saturday, killing four Marines in the deadliest attack against US troops in the country this year, officials said. The bomb in the western province of Farah targeted Marines who were helping to train Afghanistan's fledgling police force, said US spokesman Lt. Col. David Johnson. One other Marine was wounded in the attack. Marines from the 2nd Battalion, 7th Regiment based in Twentynine Palms, California, arrived in Afghanistan earlier this year and were sent to southern and western Afghanistan to train police. "The mission of the 2-7 Marines was to train and advise and mentor the Afghan National Police as part of the overall police reform strategy being employed here," Johnson said. "They're down there dedicating themselves to help create an institution that serves and protects the people of that region."