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The amphibious Ride the Ducks vehicles in Seattle are made of modern parts and are checked at least twice a day, so a malfunction that resulted in four fatalities is hard to imagine.

However, it’s not difficult to understand how a 20,000-pound vehicle could demolish a charter bus, which weighs more.

Former Duck driver and mechanic David Morrison told ³ÉÈËXÕ¾ Radio’s Ron and Don that the vehicles are solid steel. So, even though the Duck that crashed into a bus on the Aurora Bridge in Seattle on Thursday morning was probably lighter, the Duck was going to do some damage.

Related: 4 students killed in bus, ‘Duck’ collision on Seattle’s Aurora Bridge

“The bus is made of fiber glass and Duck hulls are solid steel,” Morrison explained. The Ducks are not designed to crumple, he added.

The crash that killed four people and caused at least minor injuries to at least 30 others was being investigated for several hours after. It is believed that an issue with the left-front wheel caused the Duck driver to lose control. The Duck veered from its lane and slammed into the side of the bus.

Though the crash is a tragic one, could it have been worse? Ron Upshaw wonders what would have happened if that bus wasn’t there to stop the Duck, which looked as if it were headed to the edge of the bridge.

“I don’t even want to think about it,” Morrison responded.

Though Thursday’s collision was not the first incident involving a Duck, the company has a pretty solid record, Morrison said. Drivers and mechanics adhere to a strict routine, which includes checking the vehicles before leaving for the day and after they are returned to the depot. The vehicles go through an approximate 75-point inspection to meet department of transportation and Coast Guard requirements. The check includes everything from looking at the running gear to cleaning the windshield.

At the end of the day, two mechanics check everything again, Morrison said.

Morrison worked as a driver from 2005 to 2009 and did thousands of tours. When he was driving the Ducks, Morrison said even if there wasn’t anything wrong with the vehicle, if he felt uneasy, he would switch it out for another.

“At any point during the day all the captain has to say is ‘I have a problem,’ and it’s solved,” he explained.

But even with all the precautions Ride the Ducks employees take, is it possible for a wheel assembly to come off? Don O’Neill said he’s seen a few of the vehicles broken down for various reasons.

Related: Witnesses recount ‘Duck,’ bus crash on Seattle’s Aurora Bridge

“I have not personally seen an entire wheel assembly come off,” he said. “But based on the eyewitness statements…If a wheel bearing failed, it could lead to that condition.”

Take mechanics out of the equation for a second and just think about the human factor. The drivers of the Ducks are expected to entertain, Ron pointed out. Are there incentives to do what they can for the best tips?

“Yes and no,” Morrison responded. “A good captain doesn’t sacrifice safety to make a buck.”

If that’s true, then it’s safe to say whatever happened on the Aurora Bridge was caused by a mechanical failure. Exactly what happened remains unknown at this time, however.