Radio observatories around the world pointed their dish antennas toward Mars several times over the last month to hear the signal that could have come from the $165 million probe. The lander vanished Dec. 3, just as it was beginning its descent.

"We saw something ... that had all the earmarks of a signal, and we felt we had to check it out," said Richard Cook, the lander's project manager at NASA's Jet Propulsion Laboratory. "Based on the latest results, it is unlikely that we will attempt to listen again."

The new analysis suggests the original mystery signal probably came from satellites circling Earth or another source close to home, said Ivan Linscott, an electrical engineering researcher at Stanford University.

"It does look like we're seeing something that has a higher stability than what you'd reasonably expect ... from the lander," he said.

Stanford's 150-foot antenna picked up the original signal Jan. 4, but researchers did not know about it until recorded data was analyzed in mid-January. By then, NASA controllers had given up searching for the spacecraft.

Commands were quickly beamed to Mars in an effort to make the lander -- if it was the source of the signal -- transmit again at the same frequency, but the Stanford antenna detected nothing. Other failed attempts in recent weeks involved antennas in England, the Netherlands and Italy.

Linscott said the source of the signal is not known, but the possibilities range from Earth-orbiting communications satellites to a problem with the Stanford antenna.

"We'll be looking to see if there is anything further that might help us isolate the sources," he said.

Even if researchers had been able to confirm that the signal originated with the lander, it would have been very unlikely that they could retrieve any science or images. The mission to study climate history and search for frozen water was scheduled to last through the end of February.

But investigators who are trying to figure out what happened to the spacecraft at least would have known that the probe arrived. A similarly designed mission scheduled for next year has been put on hold until the failure is understood.

A JPL investigation board is expected to release its failure report in mid-March.