TransCanada needs a permit from the U.S. government to build the section of Keystone XL planned to cross the Canadian border.
Genscape, an energy information provider in Kentucky, said it observed the pipeline's construction during an Aug. 18 flight and said it considered construction as slightly behind schedule. The company said TransCanada's assessment the project was more than 90 percent complete was "optimistic."
"Genscape now estimates first quarter 2014 to be a more realistic in-service date," it said Wednesday.
Genscape describes the Gulf Coast pipeline as a 36-inch pipeline that will stretch 485 miles from Cushing, Okla., to Nederland, Texas. It said most of the construction work left is at the Cushing end of the pipeline.
TransCanada's entire pipeline system is designated for so-called tar sands oil, a controversial heavier grade of crude oil. Keystone XL has been the target of criticism because of the perceived environmental threat from tar sands.
TransCanada had no public comment about the Genscape report.