SEATTLE — 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. A typical investigation lasts a year, Weener said.

Even before the crash, calls had emerged for greater oversight and even an outright ban on the military-style vehicles that allow tourists to see cities by road and water.