Now, environmentalists are concerned that vast troves of Keystone correspondence sent and received by Clinton might exist on private servers that will never be subject to requests for public disclosure.
"There's less risk of foul play when politicians know that anything they write is on the public record. That's why this is so troubling," said Karthik Ganapathy, a spokesman for 350.org. "People deserve to know if a Cabinet official tasked with overseeing the review process of a major project like Keystone XL, and who said she's 'inclined to approve' the pipeline, is also trading emails with the American Petroleum Institute or setting up meetings with TransCanada."
A spokesman for 350.org added that Clinton should make public any correspondence she sent and received that relates to Keystone while working at State.
Freedom of Information Act requests filed by Friends of the Earth have already turned up emails between State Department officials and lobbyists for TransCanada, the company seeking to build the pipeline, that discuss Keystone. But those requests did not uncover emails sent or received by Clinton herself.
A senior State Department official said that the department now has access to 55,000 pages of emails on an array of topics sent and received by Clinton while she served as secretary, but did not specify whether that includes communications on Keystone.
"The department has long had access to a wide array of Secretary Clinton's records—including emails between her and Department officials with state.gov"Ž accounts. Additionally, the State Department now has possession of Secretary Clinton's emails spanning her time at the State Department which are now part of the Department's records," the official told National Journal.
A spokesman for Clinton referred questions to the State Department.
And TransCanada waved away environmentalists' Keystone correspondence concerns on Tuesday.
"Over the past six and a half years, the professional activists opposed to Keystone XL have come up with one conspiracy theory after another to try and discredit a project the vast majority of Americans support. If they have any proof of their allegations, they should put that proof forward," said Mark Cooper, a spokesman for the Canadian company.
The latest criticism lobbed at Clinton from green groups highlights growing tension between the former secretary and the progressive wing of the environmental community, a key constituency that Clinton may need to help deliver a 2016 White House win.
Revelations of Clinton's use of private email arrive on the heels of reports that the Clinton Foundation pulled in millions of dollars in recent years from major oil and gas companies, including ExxonMobil and ConocoPhillips, along with a Canadian trade agency touting the Keystone XL pipeline—news that infuriated progressive environmentalists.