The former head of oil giant ExxonMobil, recused himself last month from issues related to the application for a permit for the Keystone XL pipeline's construction company TransCanada, the letter said.
"He has not worked on that matter at the Department of State, and will play no role in the deliberations or ultimate resolution of TransCanada's application," the agency’s Deputy Legal Adviser Katherine D. McManus wrote to environmental activist group, Greenpeace.
Greenpace wrote a letter to governmental officials Wednesday that asked Tillerman to recuse himself from the pipeline project because ExxonMobil could benefit from its construction due to the company's investments in Canadian oil sands.
"Secretary Tillerson's recent employer, ExxonMobil, is heavily invested in producing crude oil from Canada's tar sands and would directly and predictably benefit from the approval of TransCanada's Presidential Permit," Greenpeace wrote to the Office of Government Ethics.
President Donald Trump ordered Tillerson in January to "reach a final permitting determination" within 60 days of TransCanada's application that was filed Feb. 10.
The $8 billion, 1,900-kilometer (1,180-mile) Keystone XL pipeline will carry crude from the Canadian tar sands to U.S. refineries on the Gulf Coast. Despite congressional approval and after a seven-year review, President Barack Obama rejected the project in November 2015.