CHICAGO—The Heartland Institute, a free-market think tank, is demanding the withdrawal from the Internet of documents it says were stolen and faked.

According to a statement issued Thursday by the Chicago-based Heartland Institute, two unnamed advocacy groups posted several documents online Wednesday that they claimed were the institute's budget, fundraising and strategy plans.

Heartland's statement said the documents were obtained by an unknown person who fraudulently assumed the identity of a Heartland board member and persuaded a staff member to “re-send” board materials to a new email address.

The institute said the stolen documents appear to have been written by Heartland President and CEO Joseph Bast, but that he was traveling and had not had the opportunity to see if they were altered.

However, the statement said one document, titled “Confidential Memo: 2012 Heartland Climate Strategy,” is a “total fake apparently intended to defame and discredit the institute.”

The two-page memo says in part, “Principals and teachers are heavily biased toward the alarmist perspective” on global warming. “To counter this we are considering launching an effort to develop alternative materials for K-12 classrooms.”

It says the institute plans to pay a consultant $100,000 to produce a global warming curriculum consisting of 20 modules in 2012, with funding pledged by an anonymous donor.

The Heartland statement says, “We respectfully ask all activists, bloggers and other journalists to immediately remove all of these documents and any quotations from them, especially the fake ‘climate strategy' memo and any quotations from the same, from their blogs, websites and publications, and to publish retractions.

“The individuals who have commented so far on these documents did not wait for Heartland to confirm or deny the authenticity of the documents. We believe their actions constitute civil and possibly criminal offenses for which we plan to pursue charges and collect payment for damages, including damages to our reputation.”

The organizations cited in the Heartland statement were not identified. However, blogger Richard Littlemore, writing on the blog DeSmogBlog, where the documents are available, said in a Thursday post that it has confirmed the information in the climate strategy document by comparing its content to other material it has on hand.

Mr. Littlemore said the blog “has received no direct communications from the Heartland Institute identifying any misstatement of fact in the ‘Climate Strategy’ document and is therefore leaving the material available to those who may judge their content and veracity based on these and other sources.”