The Keystone oil pipeline ruptured in North Dakota on Tuesday morning. The spill was contained to a field of crops.

It was not immediately clear what caused the rupture or how much crude oil had spilled. Bill Suess said that an employee at the site near Fort Ransom, North Dakota Department of Environmental Quality, heard a “mechanical thud” and shut the pipeline down within two minutes.

Suess reported that oil was surfacing in a field 300 yards (274 meters) south of the pumping station. Suess reported that emergency personnel were dispatched to the scene.