The free-market Heartland Institute has moved to contain the damage from explosive revelations about its efforts to discredit climate change and alter the teaching of science in schools, claiming on Wednesday it was the victim of theft and forgery.

In its first detailed response to Tuesday's leak of documents purporting to show a well-organized campaign to cast doubt on climate change, the institute warned in a statement posted on its website: "Some of these documents were stolen from Heartland, at least one is a fake, and some may have been altered."

However, the statement from Heartland communications director, Jim Lakely, identifies only one of the eight documents posted online on Tuesday night by the DeSmogBlog website as a "total fake". That document, two pages headlined "Confidential Memo: Heartland Climate Strategy", largely duplicates information contained in the other documents.

Those documents – containing details on future projects such as a $100,000 campaign to "dissuade teachers from teaching science", as well as fundraising efforts – have been confirmed, in part, by Heartland itself, corporate donors such as Microsoft, and climate skeptic blogger Anthony Watts, who hoped to benefit from Heartland fundraising this year.

DeSmog, in its response to Heartland's charges, stood by its initial report.

"DeSmogBlog has received no direct communications from the Heartland Institute identifying any misstatement of fact in the "Climate Strategy" document," it said.