(Reuters) – TC Energy Corp has plugged its Keystone crude oil pipeline in North Dakota, allowing workers to excavate a portion of the pipeline that leaked last week and determine the cause of the spill, a state official said on Tuesday. Work will begin this week to access the precise point of the pipeline that leaked more than 9,000 barrels of crude in rural Walsh County, North Dakota, forcing the line to be shut, said Karl Rockeman, director of the state’s Division of Water Quality. There is no estimated timeline for restart of the pipeline. It was not clear how long it would take TC Energy workers and contractors to reach the affected portion of the pipe, which is buried at least four feet underground, said Rockeman, whose department is helping to oversee the cleanup work. TC Energy detected the spill on Oct. 29, forcing the company to shut the 590,000-barrel-per-day Keystone system, which transports Canadian heavy crude to refineries in the U.S. Midwest.