Story highlights Crews are testing the air near Casselton, North Dakota

They are testing for any toxic fumes

A train wreck on Monday created an oil fire

Crews in North Dakota were testing the air Tuesday morning for any dangerous fumes a day after two trains wrecked, sparking a large fire.

The results of the tests will help determine when it is safe for residents to resume their routine activities.

One of the trains was carrying crude oil, which sent dangerous smoke into the air, officials said.

The incident occurred one mile west of Casselton, a town of 2,300 residents about 25 miles west of Fargo.

Authorities issued an evacuation order, which was not mandatory, but "strongly, strongly recommended," Cass County Sheriff Paul Laney told CNN.

He estimated that 65% of area residents heeded the call to leave their homes.

There were no injuries in the wreck, and everyone is accounted for, Laney said.

"When you see the fireball and you see the damage and the aftermath, that's amazing that nobody was hurt," he said. "There were people inside their homes that could feel the heat from the explosion in their homes."

Firefighters were forced to let the fire burn out. The extreme heat made it impossible for them to get close enough to the flames to battle the blaze.