A Black Hawk helicopter crashed Thursday in southern Afghanistan, killing all four of its crew members, all of them Americans, a U.S. military official said.The crash occurred in bad weather, though the official said, "We cannot yet rule out enemy action."Troops at a combat outpost in the area waiting to be picked up saw the crash happen, the official said. Another helicopter flying nearby was not affected.Fatal helicopter crashes involving members of the NATO-led International Security Assistance Force are not unprecedented in Afghanistan, with some of the aircraft brought down by enemy fire while others crashed for mechanical reasons.The single deadliest loss for U.S. troops since the Afghan war began in late 2001 happened in August, when 30 U.S. service members died when a helicopter carrying them went down while they were reinforcing other troops, officials said. Seven Afghan troops died in that same crash.A U.S. military official said then that insurgents were believed to have shot down the CH-47 Chinook. The Taliban claimed that militants downed the helicopter with a rocket-propelled grenade.More recently, 12 people died last month when a helicopter crashed last month in Afghanistan's capital city of Kabul. Capt. David Yaryar, an ISAF spokesman, said there was no reported "insurgent activity in the area" at the time.