WASHINGTON - A helicopter has crashed at the Fort Hood military base in Texas, killing all four crew members on board, US media reported Tuesday. The accident happened after 5:49 pm (2349 GMT) Monday and emergency crews carried out an exhaustive search before locating the wreckage, NBC news reported, citing the US Army. The UH-60 helicopter was carrying members on a routine training mission and a board will investigate the crash, US media reported.