Friends of Earth said it gained access to e-mail messages that indicate State Department bias over the project.
The advocacy group points to a series of exchanges that allegedly implies a cozy relationship between TransCanada lobbyist Paul Elliot and Marja Verloop, a counselor for energy and environment at the U.S. Embassy in Ottawa. Elliot served as a campaign manager for U.S. Secretary of State Hillary Clinton during her 2008 presidential campaign.
Victoria Nuland, a spokeswoman for the State Department, said that opponents of Keystone XL are grasping at straws.
"It's obviously their First Amendment right to say what they'd like to say," she said. "We reject their accusations."
Critics of TransCanada's plans to build the pipeline to carry oil from tar sands in Alberta, Canada, to refineries along the southern U.S. coast point to everything from the corrosive nature of heavy crude to the threat to groundwater supplies as reasons to scupper the project. Backers point to its potential to bring jobs to the U.S. market and boost domestic energy security.
Friends of Earth jointed the Center for Biological Diversity and the Western Nebraska Resources Council in suing the State Department and U.S. Fish and Wildlife Service for clearing land in Nebraska in preparation of the project.
The State Department needs to approve the project because it crosses international boundaries.
Celebrity Breakups and divorces of 2014 [PHOTOS]