SEATTLE (AP) - A deadly collision last week in Seattle between a duck boat and a charter bus carrying international college students has prompted a federal investigation. Here are questions and answers about the collision and the National Transportation Safety Board probe.

SEATTLE (AP) A deadly collision last week in Seattle between a duck boat and a charter bus carrying international college students has prompted a federal investigation. Here are questions and answers about the collision and the National Transportation Safety Board probe.

WHAT HAPPENED?

A duck boat amphibious tour vehicle carrying about three dozen people and a tour bus with about 45 on board were travelling different directions across a Seattle bridge Thursday when witnesses say they saw the duck boat's left tire "lock up" as it swerved into the bus.

HOW MANY PEOPLE WERE KILLED?

Four people were killed Thursday and a fifth a 20-year-old woman died Sunday at Harborview Medical center. Her name has not been released, but she was an international student attending North Seattle College. Authorities have identified the others who were killed as: Runje Song, 17, of China; Privando Putradanto, 18, of Indonesia; Mami Sato, 36, of Japan; and Claudia Derschmidt, 49, of Austria. All were international students from North Seattle College. The students were heading to new student orientation activities.

A total of 51 people were transported to area hospitals following the accident. Thirteen remained hospitalized Sunday.

WHAT HAS THE INVESTIGATION FOUND?

Authorities say it will be months before they determine an exact cause of the accident. They say they know the duck boat's left front axle was sheared off, though it wasn't clear if the axle had broken before or after the collision. NTSB Board Member Earl Weener said Sunday the duck boat involved in the collision did not have an axle repair that was recommended for at least some of the amphibious vehicles in 2013. Weener said Ride the Ducks International, which refurbished the boat in 2005, issued a warning to its customers two years ago about potential axle failure and recommended a specific repair or increased monitoring. It's unclear if the company that owns the vehicle Ride the Ducks of Seattle was aware of the warning, Weener said.

In a statement Sunday night, Ride The Ducks of Seattle owner Brian Tracey didn't say whether the company knew of the warning. "We are working to understand what happened, and have completely opened our operations to NTSB investigators," he said.

The vehicle involved in the Seattle crash was an Army surplus craft built in 1945.

WHAT EXACTLY ARE DUCK BOATS?

The U.S. Army deployed thousands of amphibious landing craft during World War II that were known then by their military designation, DUKW. Once the war was over, they became used by civilian law enforcement agencies and also converted to sightseeing vehicles in U.S. cities. The DUKW designation was replaced with the duck boat moniker that is used by various tour companies.

HAVE THEIR BEEN PREVIOUS ACCIDENTS WITH DUCK BOATS?

Thirteen people died in 1999 when an amphibious boat sank to the bottom of Lake Hamilton in Arkansas in an accident the National Transportation Safety Board blamed on inadequate maintenance. Investigators determined that the vessel, built by the Army in 1944, was not designed for passenger service and as a result lacked the proper buoyancy to remain afloat.

Two Hungarian tourists were killed in 2010 when a sightseeing duck boat was hit by a barge on the Delaware River near Philadelphia, sinking it in water about 55 feet deep. More than 25 people were injured. The NTSB found that the tugboat operator was distracted by communicating with family members on his cellphone and laptop computer. Investigators also found fault with the maintenance of the duck boat and decisions by the captain to anchor in an active navigation channel.

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