Flight controllers in Moscow were able to re-establish some communication with the computers overnight, and Russian engineers were working Thursday to restore the rest of the system, Nasa space station flight director Holly Ridings said.

"They've made a lot of progress," she said. "There are some cleanup steps to do still and some investigation."

Officials with Nasa and the Russian space agency still don't know why the computers went down. They had never seen that type of failure on the space station before, and they believe it may be related to electrical power rather than computer software.

A new solar array had been unfolded outside the station Tuesday to help provide power for the orbiting outpost, and astronauts spent Wednesday hooking up a joint that will let the solar arrays track the sun.

The crew got a scare early Thursday while the computers were being reconnected: A false fire alarm went off on the Russian Zarya module, but Ridings said there was no indication of any fire or smoke.

Atlantis is still docked at the space station, so the astronauts periodically fired its thrusters to help maintain the space station's position while the computers were down. Nasa had said it might have to extend the shuttle's mission because of the problem; the mission already had been extended from 11 to 13 days to repair a thermal blanket that peeled up during launch.

In a worst-case scenario, Nasa said, the space station's three crew members might have to return to Earth early if the computers aren't fixed. Without the Russian oxygen-machine running, the space station has a 56-day supply of oxygen left.

The space station is operated primarily by the Russian and U.S. space agencies, with contributions from the Canadian, European and Japanese space agencies.