Astronauts returned to the U.S. side of the International Space Station on Wednesday, hours after an apparent false alarm raised concerns about an ammonia leak and forced a partial evacuation.

The six crew members shut down the U.S. segment and hung out on the station's Russian side for much of the day while ground controllers went about troubleshooting the problem. Eleven hours after the alarm arose, mask-wearing astronauts went back in and sampled the air.

"No ammonia indication," NASA reported in a tweet.

The likeliest cause of the alarm was a malfunctioning card in a signal-processing box, NASA space station project manager Mike Suffredini said during a televised update.

"At this point, the team does not believe we leaked ammonia. ... What we are dealing with is a failure, probably of a card inside a multiplexer-demultiplexer," he said. The space station has a number of multiplexer-demultiplexer boxes that process readings from components aboard the orbital outpost, which has as much livable space as a six-bedroom house.

NASA said turning the box off and then back on cleared up the worrisome readings.

The alarm was raised at 4 a.m. ET, when Mission Control saw pressure changes that could have been caused by an internal leak in the station's coolant system, which uses water on an inside loop and toxic ammonia on an outside loop. "If this is possible, then we immediately 'safe' the vehicle and get the crew in a safe place," Suffredini said.

Crew members put on emergency masks, powered down the systems on the station's U.S.-built segment, moved into the Russian segment and closed a connecting hatch. The ammonia coolant system services only the U.S. side of the station, so controllers said there was no risk to the crew.

"The safety of the team was preserved thanks to swift actions of the cosmonauts and astronauts themselves and the team on the ground in Moscow and Houston," said Maksim Matyushin, the chief of Russia's Mission Control.

Follow-up readings indicated no sign of an actual ammonia leak into the station, although fluctuations in cabin pressure continued to cause concern. Suffredini said those fluctuations were probably the system's "normal reaction to the events that started to unfold."

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Space Station Crew Safe After Partial Evacuation: NASA