The National Transportation Safety Board dispatched a team Friday to begin its investigation into a deadly collision involving an amphibious bus, one of the “duck boat” attractions that carry sightseers around major cities by road and water.

Four persons died and 15 were hospitalized after the Thursday afternoon crash, most of them students from North Seattle College riding a chartered bus, according to Seattle police. The bus and the duck boat were heading in opposite directions on the Aurora Bridge over Lake Union when the duck boat suddenly veered into the oncoming lanes, witnesses told police.

The duck boat hit the side of the bus, spilling passengers into the roadway, and the bridge became a mass casualty scene as emergency workers gave first aid and began transporting victims to hospitals.

Investigators are likely to look closely at the mechanical condition of the duck boat, based on witness statements that the vehicle fishtailed and may have lost a front wheel just before the impact.

The six-wheeled vehicle was operated by Ride the Ducks of Seattle, one of several operators in tourist-friendly cities including Boston and Philadelphia.

The boats get their name from the World War II DUKW 31’x8’ amphibious trucks, dubbed “ducks” back then by American soldiers. Some 21,000 were built, and in postwar years many were repurposed as water rescue vehicles by the Coast Guard, fire departments and other agencies, and others upgraded as tourist attractions.

Today, new commercial duck boats are purpose-built, and all are subject to both commercial passenger vehicle regulations and Coast Guard rules. The Seattle company, in business since 1997, stresses safety concerns to customers on its website: “All our Captains hold a United States Coast Guard Master’s license, as well as a commercial driver’s license, are CPR and First Aid certified and must complete a rigorous training program before any paying-passengers board their Duck. Our fleet of DUKWs is annually inspected by the USCG and bi-annually by the DOT and our paperwork files are audited frequently by both organizations.

“There are redundant safety systems onboard and our operations manual has been used as a prototype for amphibious tours around the country. Our goal is always to exceed the requirements of regulatory agencies and set the standard for others to follow.”

The NTSB has investigated two other major casualties involving duck boats on the water: A 2010 collision in the Delaware River at Philadelphia that left two Hungarian tourists dead, and a 1999 sinking on Lake Hamilton in Hot Springs, Ark. where 13 died.

In the 2010 Philadelphia case, the 79’ 2,400-hp tugboat Caribbean Sea operated by K-Sea Transportation Partners was moving the empty 250’ sludge barge The Resource when it collided with the DUKW 34, whose captain had anchored in the channel after the engine overheated.

NTSB investigators determined the tugboat mate was inattentive because he was using a cell phone and laptop computer to communicate with his family while they dealt with a family emergency. Instead of navigating from the upper wheelhouse, the mate was in the lower wheelhouse with restricted visibility.

The safety board’s 2011 report reverberated in the industry with the realization distracted operators could be a danger. At the time board member Robert Sumwalt, who had been the NTSB’s on-scene spokesman, said electronic device distractions “are becoming the new DUI…It’s going to reach epidemic proportions.”

The agency also faulted operator Ride the Ducks International LLC for its mechanics’ failure to secure a surge tank pressure cap on their vehicle, which led to the engine overheating and the captain anchoring without following all safety procedures such as having passengers don life vests.

The 1999 Hot Springs case involved an original Army DUKW built in 1944, converted to the tour boat Miss Majestic. Investigators determined the vessel flooded through a loose rubber boot on a through-hull drive axle, and the sinking was hastened because the old wartime DUKW did not have adequate reserve buoyancy.

Beside faulting operator maintenance, the NTSB found the Coast Guard did not have uniform standards for evaluating commercial amphibious vehicles, and recommended steps including retrofitting with watertight compartments and removing canopies that could hinder escape in emergencies.