TransCanada shut down its 7-year-old Keystone Pipeline on Thursday after an estimated 5,000 barrels of oil—some 210,000 gallons—spilled across grassland near a pump station in South Dakota. The spill occurred as regulators in Nebraska are preparing to decide on Monday whether to allow TransCanada to build the new Keystone XL pipeline across their state.

The pipeline company reported that the spill was discovered after a drop in pressure was detected and said that the oil was isolated quickly.

TransCanada didn’t say how long the pipeline—which carries tar sands oil from Alberta, Canada, to Oklahoma and to Illinois—would be shut down or what had caused oil to spill.

“We’ve always said it’s not a question of whether a pipeline will spill, but when, and today TransCanada is making our case for us,” said Kelly Martin, a campaign director for the Sierra Club. “This is not the first time TransCanada’s pipeline has spilled toxic tar sands, and it won’t be the last.”

The Natural Resources Defense Council pointed out that this was the pipeline’s third major spill in the region, following a 21,000-gallon spill in its first year (one of at least 14 leaks that year) and a 16,800-gallon spill last year.

“This spill should be a stark warning for Nebraska’s PSC (Public Service Commission) as it considers TransCanada’s proposed route for Keystone XL through some of the state’s most sensitive farmlands and aquifers,” wrote Anthony Swift, Canada Project Director for NRDC.

On Monday, the Nebraska Public Service Commission is expected to issue a decision on whether to permit construction on the next phase of TransCanada’s Keystone system—the northern leg of Keystone XL. The expansion would have the capacity to pump more than 800,000 barrels of tar sands crude oil a day from Alberta to Steele City, Nebraska, and then on to refineries on the Gulf Coast through connecting pipelines.

The state commission is the last regulatory hurdle for a project that has drawn protests and lawsuits since it was proposed.

The Nebraska commission has been hearing concerns from landowners and indigenous groups who worry about spills and construction damage to their property. The commission’s task is fairly narrow, however: It is to consider whether the new 1,180-mile pipeline is in the public interest. During a week of hearings in August, that did not include issues of safety or actual need for the pipeline.

The Keystone XL project was proposed in 2008. The southern half of the project was built and became operational before President Obama stopped the upper leg in 2015. President Trump, shortly after he took office in January, encouraged the pipeline company to resubmit its permit request and issued an executive order directing his administration to expedite it.

While approval from the commission could clear the way for the pipeline, market demand will still play into whether the Keystone XL pipeline moves forward. A global oil glut has dropped prices, there is ample supply of lighter crude from the U.S. Bakken reserves, and several large oil companies have pulled out of the Canadian tar sands. TransCanada told financial analysts in July that it would determine whether it had the customer base to move forward with the project.