SEATTLE (AP) — The National Transportation Safety Board's investigation into the deadly crash of a duck boat and charter bus is the first time the agency has looked into a land crash of the amphibious vehicles critics say are too dangerous for city streets.The agency has scrutinized the military-style vehicles several times when they've been in accidents on water, board member Earl Weener said at a news conference Friday.Four international students died in the crash Thursday after witnesses said the duck boat veered into the oncoming bus on a Seattle bridge.A team of investigators arrived Friday and would spend a week or more on site.