CAPE CANAVERAL, Fla–Astronauts have returned to the American section of the International Space Station after alarms indicating a possible ammonia leak forced them into the Russian segment twice.

NASA station managers have issued an update directing the space station residents to return by the end of the day on Wednesday. No ammonia was detected in the cabin and all crew members remain safe.

The Johnson Space Center in Houston continues to evaluate data from the incident. Michael Suffredini, manager of the International Space Station Program, believes a “computer card” could be responsible for the false alarm.

Some crew members returned at  3:05 p.m. Eastern Time wearing the protective masks they had been issued following the alarm. They are taking air samples as a safety precaution. Their masks will be removed if a lack of ammonia is reconfirmed.

Systems in the cabin indicated measurements appeared to be off scale at about 4 a.m. Wednesday. The six Expedition 42 members immediately donned protective masks and sealed themselves in the Russian segment. They were permitted to remove their masks once it was confirmed to be free of ammonia.

Teams reviewed the data in Houston and instructed the astronauts to return to the American segment but a pressure increase, possibly caused by leaking ammonia, forced them to don their masks and reseal themselves in the Russian segment.

“We also saw an increase in pressure in the cabin,” said Suffredini. “If you’re leaking ammonia into the water loop and it finds its way into the cabin, you’d expect its pressure to go up.”

The International Space Station’s cooling system uses ammonia in the exterior loops that transfer heat from the inside. Because ammonia is toxic, water is used inside to transfer the heat to the loops on the outside.

The station is manned by NASA astronauts Barry Wilmore and Terry Virts, Russians Elena Serova, Alexander Samoukutyaev and Anton Shkaplerov, and European Space Agency astronaut Samantha Cristoforetti.

NASA hopes the team will resume activity Thursday.