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.

The Keystone Pipeline in North Dakota was shut down Tuesday after reports of a "bang" and spilled oil, according to officials.