Last Updated Jan 14, 2015 4:58 PM EST

Crew members returned safely to the U.S. segment of the International Space Station Wednesday afternoon after evacuating earlier in the day.

Concern about a possible ammonia coolant leak early Wednesday prompted the three astronauts to evacuate their portion of the complex, joining three cosmonauts in Russian modules while flight controllers studied telemetry to figure out if alarms were triggered by an actual leak, a sensor problem or some other issue, officials said Wednesday.

A few hours later, engineers studying telemetry found no evidence of any leaking ammonia, raising suspicion that a circuit board, or card, in a specific computer known as a multiplexer-demultiplexer, or MDM, might have suffered a failure that took four critical sensors off line. That, in turn, triggered a sequence of events that may have combined to indicate a leak in the station's ammonia coolant system.

"At this point, the team does not believe we leaked ammonia," Mike Suffredini, the space station program manager, said at 11 a.m. EST (GMT-5). "What we are dealing with is this failure of probably a card inside one of our multiplexer-demultiplexers, it's just a computer that sends telemetry down and brings commands back up. This card has a number of measurements on it and those were the measurements we lost."

The alarms were triggered just after 4 a.m. EST (GMT-5) when an apparent pressure increase was detected in a water coolant loop in the forward Harmony module. Water is circulated inside the station to carry away the heat generated by the lab's electronics. The water then flows through components called heat exchangers, transferring the heat to ammonia coolant that flows through huge external radiators to keep the station within temperature limits.

The apparent pressure spike in coolant loop B was a possible indicator of an ammonia leak, and playing it safe, flight controllers told Expedition 42 commander Barry "Butch" Wilmore, Terry Virts and European Space Agency astronaut Samantha Cristoforetti to don masks and move into the Russian segment of the space station.

The telemetry was confusing at first and the initial evacuation was briefly called off. But when flight controllers saw indications of a slight pressure increase in the crew's air supply -- a more convincing sign of an actual leak into the station -- they told the crew to head back to the Russian segment.

Joining cosmonauts Alexander Samokutyaev, Elena Serova and Anton Shkaplerov, Wilmore, Cristoforetti and Virts closed a hatch between NASA's Unity module and the Russian Zarya module, isolating all six crew members in the Russian segment of the lab complex, which uses a different cooling system.

Flight controllers then powered down critical systems tied into coolant loop B and shut down an external pump to reduce the pressure in the system. After studying the telemetry, however, engineers saw signs that something in that powerdown process might have triggered the slight change in air pressure that was detected.

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U.S. part of space station evacuated