Workers have recovered thousands of gallons of crude oil from an underground pipeline spill on North Dakota farmland, the owner of the line said Thursday, but it remains unclear when oil will again start flowing to refineries.

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.

Officials do not know how much oil leaked from the Keystone Pipeline after an employee heard a loud noise Tuesday morning.