A “Ride the Ducks” land-water tour vehicle—usually known for careening around major cities in a zany manner while tour groups blow duck whistles at pedestrians—met with disaster when it collided with a charter tour bus around 11 a.m. Thursday on Seattle’s Aurora Bridge. There were 50 injuries—12 of them critical—and four fatalities.

The cause of the crash is officially still unknown, but a witness told KIRO-TV that the duck’s driver “overcorrected,” swerving across traffic and slamming into the bus traveling the opposite direction. A Seattle Fire lieutenant said bus passengers suffered the most severed injuries, and the people killed were likely on the bus, not the duck.

Seattle mayor Ed Murray said foreign students were involved, and their consulates are being contacted, according to Seattle Times reporter Christine Clarridge.

Two passenger vehicles were also involved, a Seattle Fire spokesperson told NBC News.

As of 12:30 p.m., all the injured passengers had been cleared and another bus had arrived to take people away from the area. The bridge, one of only a few major north-south routes in the city, won’t reopen until late Thursday night at the earliest.

Update: The number of critical injuries is up from nine to 12, according to Mayor Ed Murray.

North Seattle College has also issued a statement saying the charter bus was carrying students from its international program, according to NBC News.

Local NBC affiliate KING-5 adds this harrowing account of the crash from a witness:

“The duck boat was signaling to enter the left lane, and as it was making its turn to enter the left lane, it seemed to lurch suddenly.

I saw a bunch of smoke and what appeared to be the front left wheel pop off, and then it clipped a smaller SUV and basically almost T-boned into the oncoming bus and spun around.”

[Photo: KIRO-TV/Twitter]