(CNN) –Union Pacific crews are readying to remove rail cars after a train derailment and fire prompted evacuations in northwest Iowa on Sunday, Union Pacific spokesperson Robynn Tysver said Monday.

“We know the impacted cars were carrying hydrochloric acid, potassium hydroxide and asphalt,” Tysver said in an email to CNN.

The derailment happened at around 2 p.m. Sunday in Sibley and involved an estimated 47 cars, Tysver told CNN affiliate KMEG. The train crew was not injured, Tysver added.

Voluntary evacuation orders remain in place and the Sibley-Ocheyedan Community School District is operating on a late start schedule, the district said in a Facebook post.

The cause of the derailment is under investigation, Tysver said. Authorities said the fire was caused by diesel fuel, according to CNN affiliate KTIV, and that the derailment was a result of a bridge collapse.

For people who didn’t evacuate, the Osceola County Sheriff’s Office wrote in a Facebook post that residents were “advised to open windows on both sides of their homes and be positioned in the middle of their homes.”

“This is a safety measure in case a rail car explodes,” the sheriff’s office wrote.

All waterways in the area were blocked off, KTIV reported.

CNN has reached out to the sheriff’s office and the county emergency management for more information.