The Environmental Protection Agency has issued a surprising public challenge to a State Department impact study that effectively endorsed the Keystone XL pipeline. Released last month, the long-awaited assessment concluded the Keystone XL does not threaten the global climate and would lead to fewer emissions than critics have alleged. The State Department’s review marked a major boost to the Keystone XL pipeline’s chances as President Obama mulls whether to approve or reject it. But in an unprecedented letter, the EPA said the State Department had used “insufficient information” to reach its conclusion. The EPA letter singles out the review’s conclusion that building the pipeline will not adversely affect climate change nor impact whether tar sands oil is extracted. The letter was submitted as part of the project’s public comment period, which ended on Monday, Earth Day. Environmentalists say the comment period yielded more than one million submissions opposing the pipeline’s construction.