Trucks and workers started a cleanup effort at the site of a spill of the Keystone oil pipeline in rural North Dakota

The cause of the rupture and the total volume of oil released after the North Dakota spill remain unclear, officials said.

The nearly 2,700-mile Keystone oil pipeline was shut down Tuesday morning after it ruptured in North Dakota, halting the flow of millions of gallons of crude oil from Canada to refineries in the U.S.