keystone pipeline

The cause for the spill is still unclear. What's known is the type of oil that was piped: tar sands oil, also called diluted bitumen. It's a lot harder to clean up than conventional oil.

The leak was initially estimated to have released some 210,000 gallons of crude. Federal investigators say that they now have an "unconfirmed lower spill estimate," without specifying how much.

The oil leak occurs just a few days before Nebraska state regulators will decide on the fate of TransCanada's controversial sister project, the Keystone XL Pipeline.