Gunmen mounted on pick-up trucks and motorcycles killed 12 paramilitary police and wounded several in an attack on their base in restive southwest Niger, near the Mali border, on Saturday, two security sources said. The village is a few dozen kilometers from where militants killed four US soldiers in an ambush on Oct. 4 that has thrown a spotlight on the US counter-terrorism mission in Niger, which straddles a large expanse of the Sahara. The gunmen crossed over the border from Mali and drove up to the village of Ayorou, before launching their attack, the security sources said. "They were heavily armed. They had rocket launchers and machine guns. They came in four vehicles each with about seven fighters," said a security source on the scene. One of the attackers was killed in an exchange of fire, he added. A spokesman for Nigers military said he could not confirm any details of the attack. Several militant groups and well-armed ethnic militia are known to operate in the area near the border with Mali, and there have been at least 46 attacks recorded there since early least year. However, security officials suspect a relatively new militant group called ISIS in the Greater Sahara to have been behind many of them, including the ambush on the joint US-Niger patrol. Meanwhile, members of Congress are demanding answers to the killing of the four US soldiers. Among the unresolved inquiries: Why were the Americans apparently caught by surprise? Why did it take two additional days to recover one of the four bodies after the shooting stopped? Was ISIS responsible? The confusion over what happened in a remote corner of Niger, where few Americans travel, has increasingly dogged President Donald Trump, who was silent about the deaths for more than a week.