“We are aware of an incident involving Amtrak train 501,” the rail operator wrote on Twitter. “We will update with additional details as they become available.”

First responders are treating the derailment as a “mass casualty incident,” according to local news outlet KIRO 7.

Washington Governor Jay Inslee has tweeted a “thank you” to the first reponders at the scene, while referring to the incident as an “ongoing and serious situation.”

Amtrak train 501 was part of a service that launched Monday morning, according to CBS News.

“We had just passed the city of DuPont and it seemed like we were going around a curve,” Karnes said, as quoted by CBS News. “All of a sudden, we felt this rocking and creaking noise, and it felt like we were heading down a hill. The next thing we know, we’re being slammed into the front of our seats, windows are breaking, we stop, and there’s water gushing out of the train. People were screaming.”

“The tracks for this line were supposed to be upgraded to be able to handle higher speeds,” Karnes continued. “I’m not sure what happened at this juncture.”