Officials with the U.S. space agency, NASA, say political appointees within the agency's public affairs office deliberately "marginalized or mischaracterized" studies of global warming.
In a report released Monday, NASA's inspector general called the appointees' actions "inappropriate political interference."
The report evaluated allegations that from 2004 through 2006 NASA's public affairs office downgraded press releases having to do with global warming or denied media access to scientists.
But the report also said investigators did not find any credible evidence that senior NASA or Bush administration officials directed the effort.
NASA representatives said the agency has already revised its policy for dissemination of science information after allegations made in 2006.
Separately Tuesday, lawmakers in the U.S. Senate continued discussions on climate change legislation.
The bill, which has the support of Democratic lawmakers and some Republicans, aims to reduce the amount of carbon dioxide and other greenhouse gases by two-thirds by the year 2050.
President George Bush has threatened to veto the bill, saying it would impose some $6 trillion in new costs.
President Bush has said he opposes mandatory limits on greenhouse gas emissions, saying it would put U.S. businesses at a competitive disadvantage with foreign countries. Mr. Bush says global warming can be addressed through voluntary limits and new energy technologies.
Some information for this report was provided by AP.