NASA's press office "marginalized or mischaracterized" studies on global warming between 2004 and 2006, the agency's own internal watchdog concluded.
In a report released Monday, NASA's inspector general office called it "inappropriate political interference" by political appointees in the press office.
It said that the agency's top management wasn't part of the censorship, nor were career officials.
NASA downplayed the report as old news on a problem that has since been fixed. NASA spokesman Michael Cabbage said the space agency's new policies have now been hailed for openness by the U.S. Government Accountability Office.
The report found credence in allegations that National Public Radio was denied access to top global warming scientist James Hansen.
It also found evidence that NASA headquarters press officials canceled a press conference on a mission monitoring ozone pollution and global warming because it was too close to the 2004 presidential election.
In addition, the report detailed more than a dozen other actions in which it said the NASA public affairs office unilaterally edited or downgraded press releases having to do with global warming or denied access to scientists.
NASA public affairs officials criticized by the report called it wrong, saying they were always open and truthful.
Not so, according to the report. The report did not directly accuse them of lying, but used more nuanced terms such as "mendacity" and "dissembling."
The space agency complained those terms were unjust.
The report concluded that "inappropriate political posturing or advantage" was behind some of these actions.
NASA and the Bush administration instantly drew criticism as a result of the report.
"Our government's response to global warming must be based on science, and the Bush administration's manipulation of that information violates the public trust," said Sen. Frank R. Lautenberg, D-N.J.
Mark Bowen, who wrote a book on NASA and Hansen, faulted the report's finding that NASA administrator Michael Griffin and the White House weren't involved in manipulation.
"So many honest people inside NASA and out have demonstrated censorship has occurred," Bowen said.
In its response, NASA's legal office noted that the report showed that actual research on global warming was not interfered with, and that neither NASA senior management nor other senior administration officials were involved.
"The legitimate conclusions ... are those that NASA has already acknowledged and has long since fixed," deputy counsel Keith Sefton wrote in response.
NASA's former press secretary, Dean Acosta, who was accused of telling underlings that there were "too many" global warming news releases, denied manipulations.
"My entire career has been dedicated to open and honest communications," said Acosta, now a spokesman for aerospace giant Boeing Co. "The inspector general's assertions are patently false."
NASA's overall head of public affairs, David Mould, who was also criticized, said the report "got a number of things wrong... I didn't see things that were politically influenced."
But Mould also pointed to changes in policy he made after the allegations first came out in 2006, saying "I'm proud of the improvements we made."