NATO officials said Monday that Taliban forces in Afghanistan fired a rocket that downed a helicopter and killed 38 people last Friday. It was the worst single incident of the decade-long war for the U.S., with 30 American troops dead. Some of them belonged to the same Navy SEAL unit that killed Osama bin Laden in May, although none of the dead had actually participated in that operation. Seven Afghan commandos and an Afghan interpreter were also killed in the crash. The NATO-led International Security Assistance Force said the CH-47 Chinook helicopter was fired upon by a rocket-propelled grenade while the chopper was transporting the unit to “the scene of an ongoing engagement.” The troops were searching for a Taliban leader in the rugged terrain of the Tangi Valley, about 80 kilometres southwest of Kabul. The incident is one of the latest to underscore the escalating violence in the war-torn country. The first six months of 2011 has seen record levels of civilian casualties since a U.S.-led coalition invaded Afghanistan in 2001.