(Bloomberg) -- House Republicans have launched an investigation into six environmental organizations over their opposition to a $9 billion oil project developed by ConocoPhillips in Alaska.

In letters made public Friday to the Center for Biological Diversity, the Defenders of Wildlife and other conservation groups, lawmakers said they were concerned about “coordination with other activist special interest groups” as well as the use of “lawfare strategies, including ethically dubious sue and settle tactics.”

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The probe follows unsuccessful legal efforts to halt ConocoPhillips’ Willow project, which is expected to produce about 600 million barrels of crude over 30 years, with production set to begin in 2029.

“Given Greenpeace’s questionable litigation strategies and the importance of Willow to Alaskan economic prosperity and American energy security, the Committee is keen to uncover the details of Greenpeace’s opposition to the project,” Representative Bruce Westerman, chairman of the House Committee on Natural Resources, wrote in the letter. Similar notices were also sent to Earthjustice, Friends of the Earth and the Natural Resources Defense Council.

“Throughout our litigation, and multiple court rulings in our favor, we have presented evidence that the approval of the Willow project violated bedrock laws and did not properly consider the detrimental harms to our climate, wildlife, and people’s livelihoods that will be directly impacted,” Drew Caputo, Earthjustice’s vice president of litigation for lands, wildlife, and oceans, said in an emailed statement. Friends of the Earth issued a similar statement.

“We will continue the very American tradition of using free speech and environmental laws to protect the Arctic and all American public lands threatened by corrupt politicians and the oil industry,” the the Center for Biological Diversity wrote in letter responding to the lawmakers query on Friday.

The Natural Resources Defense Council declined to comment. ConocoPhillips, Defenders of Wildlife and Greenpeace didn’t respond to a request for comment.

Lawmakers are seeking documents and communications between the environmental groups and several government agencies, including the Interior Department and the National Oceanic and Atmospheric Administration. They are also requesting communications between the groups and nationals from Russia, China, Iran or North Korea.

A committee spokesman didn’t respond to a request for more information on why those communications were being sought. Republicans have previously accused environmental groups of accepting funds from organizations linked to the Chinese Communist Party and Russia to finance advocacy efforts that hinder domestic energy production. The organizations have denied the claims, calling them a smear campaign backed by the fossil fuel industry.

“This GOP congress, which has not had a single serious oversight hearing or investigation about the lawless, destructive chaos of the Trump administration over the past year, jumps at every opportunity to bully, intimidate, and harass anyone who challenges the administration’s massive giveaways to Big Oil,” Representative Jared Huffman, the top Democrat on the Natural Resources Committee, said in a statement.

(Updates with additional comments from Friends of the Earth and Jared Huffman.)

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