WASHINGTON, Jan. 14 -- Astronauts returned to the U.S. segment of the International Space Station Wednesday afternoon after a false alarm prompted an emergency evacuation to the Russian side, U.S. space agency NASA said.

"The crew opened the hatch to the U.S. segment and returned inside at 2:05 p.m. Central time (1805 GMT)," NASA said in a statement.

Wearing protective masks, astronauts Terry Virts of NASA and Samantha Cristoforetti of the European Space Agency "sampled the cabin atmosphere and reported no indications of any ammonia," it said.

The U.S. side of the station sounded an alarm at about 4 a.m. EST (0900 GMT), indicating a possible ammonia leak in the cooling system. U.S. station commander Barry Wilmore, flight engineers Virts and Cristoforetti were directed to don protective masks and move into the Russian segment, closing the hatches between the U.S. and Russian segments.

Non-essential equipment in the U.S. section was powered down. At the same time, the station's protection software shut down one of two redundant cooling loops known as Thermal Control System Loop B.

But in an update Wednesday afternoon, NASA said data received from a variety of system sources on board indicated no leakage of ammonia on the station.

The space agency said the alarm this morning was suspected to have been caused by a transient error message in one of the station's computer relay systems, called a multiplexer- demultiplexer.

"A subsequent action to turn that relay box off and back on cleared the error message and the relay box is reported by flight controllers to be in good operating condition," it said.

The six astronauts aboard the station, also including Russians Elena Serova, Alexander Samokutyaev and Anton Shkaplerov, were never in any danger, NASA said.

Cristoforetti, who is from Italy, said via Twitter: "Hey everybody, thanks for your concern. We're all safe & doing well."

Flight controllers are continuing to analyze data in an effort to determine what triggered the alarm, and the astronauts are expected to resume normal research activities on Thursday, NASA said.