But Sen. John McCain (R-Arizona), on his first day back at the Senate following a failed presidential campaign, expressed alarm at the double failures of last year's Mars Climate Orbiter and the Polar Lander. McCain also warned that any cover-up could damage NASA's credibility.

"UPI never called us and never asked us about this before putting it out. The suggestion that we knew about [problems with the Polar Lander] and didn't tell the public about it is just plain wrong."

"If the media reports are true, then the trust between government and its citizens has been violated," McCain said.

The Arizona Republican, who is also chairman of the Senate Committee on Commerce, Science and Transportation, said NASA's management needs more congressional oversight.

Reports of a NASA cover-up appeared on several internet websites, aired overnight on radio news broadcasts and made the pages of a number of morning newspapers. They originated from the UPI wire story that quoted an unnamed source "close to" the Mars Polar Lander investigation panel.

The story outlines two possible design flaws that could have contributed to a lander crash when the spacecraft reached Mars last December 3. It alleges that officials covered up these fatal flaws.

A statement issued by the Jet Propulsion Laboratory (JPL) Tuesday calls that story, "at varying times speculative, incorrect and shows a seriously garbled understanding of the mission and its engineering and management history. The full report on the loss of the Mars Polar Lander will be released next week and contain all the facts as we know them to be."

A panel led by former Lockheed Martin executive Thomas Young prepared that report. It was completed and delivered to NASA March 15.

In Washington Wednesday, NASA spokesman Brian Welch vehemently denied the UPI report, calling it "irresponsible and inaccurate."

"UPI never called us and never asked us about this before putting it out," Welch said. "The suggestion that we knew about [problems with the Polar Lander] and didn't tell the public about it is just plain wrong."

Welch pointed to a November 10 report issued by Art Stephenson, director of the Marshall Space Flight Center in Huntsville, Alabama, which investigated problems with an earlier Mars failure -- the Mars Climate Orbiter. In his report, Stephenson warned that problems might arise with the Polar Lander's braking rockets because the fuel lines to those rockets could malfunction due to cold conditions in space.

"We knew about, and we took corrective action to prevent that," Welch said. Engineers at JPL made "a number of recommendations that were in Stephenson's report relating to that issue."

NASA officials also denied UPI's allegation that a manager at Lockheed Martin purposely altered the conditions of a preflight test of the Polar Lander so that the spacecraft would pass inspection.

What actually happened, NASA said, was that Stephenson's review board found that the Polar Lander's systems had been tested at room temperature -- not the subfreezing temperatures it would encounter en route to Mars.

That's when the review board advised Lockheed Martin to study how the system might perform at cold temperatures and, as a result, recommended that heaters on the spacecraft's engine be turned on earlier than originally planned in preparation for its arrival at Mars.

A second issue raised in the UPI report -- that the probe's braking rockets might have shut off prematurely, causing the craft to crash -- "is among the failure modes" addressed in the Young report, Welch said.

SPACE.com reported both these possible problems in November and February, respectively.

Last month, speculation centered on the possibility that a mis-set switch may have sent an incorrect signal to the lander's software, causing the craft's engines to shut down prematurely, causing it to crash on Mars. One JPL engineer told SPACE.com that the potential for such a failure had been identified by reviewers in the months following the lander's demise, and that it could have been prevented. The software error, he said, was "a really stupid mistake," and one that "any freshman programmer" could have avoided.

According to several Polar Lander engineers, there is a long list of potential causes for the lander's failure, but none has yet been named as a "smoking gun." Sam Thurman, the mission's flight director, told SPACE.com that even the completed report relies to some degree on speculation because the lander sent back no data after it began its descent into Mars' atmosphere. Thurman said he hopes to "urge a cautionary note" on the interpretation of the review board's findings.

The story of the Polar Lander's failure goes deeper than what is discussed in the Young report, said Pat Dasch, executive director of the National Space Society, a space advocacy group in Washington.

"Congress will be asking probing questions about this and we have to get to the bottom of it. Right now, you're hearing serious accusations and outright denial. Somebody's got to be wrong."

While the failures are "a black eye" for NASA, Dasch said, "I'm sure the agency can bounce back. It's going to take some work and some reforms but I think it will be stronger for it. Mars exploration is important to the American people and this program will move ahead, but it needs some reworking. The bottom line in all this is, you may be able to do programs faster, better and cheaper, but you can't do cutting-edge exploration on the cheap."

An official NASA response to the UPI story, which the agency posted prominently on its website, said members of other review teams convened to look into the problems are using words like "bunk," "complete nonsense" and "wacko," to describe the substance of the wire report.

The fact that NASA has responded so strongly is surprising. Typically, the agency will withhold comment about investigations until the official report is released.

More news may come out of an afternoon session of Congress Wednesday when members of the Senate Science Technology and Space Subcommittee question NASA Administrator Dan Goldin, and other NASA officials who have led recent review panels.

SPACE.com's Andrew Bridges, Andrew Chaikin and States News Service's Alex Canizares contributed to this story.