Astronauts at the International Space Station rushed to take shelter after a system failure signalled a possible ammonia leak, but NASA said later it was likely a false alarm.
The six-member crew donned emergency masks and hurried to the Russian side of the orbiting lab two separate times after the problem was noticed at around 4am on Wednesday.
At first, it seemed that high pressure outside the space station could have led to ammonia leaking inside, so the crew put on breathing masks and moved to the Russian side for safety.
Ammonia is used in the cooling and heating systems at the orbiting outpost.
The crew was allowed to come briefly back to the US side, but when NASA noticed that pressure in the cabin was rising, they returned to the Russian side and closed the hatch behind them.
While the Russian space agency told news outlets in Moscow that the cause was a toxic ammonia leak, NASA said there was no data to confirm that, and stressed that the crew was safe.
"At this time the team does not believe we leaked ammonia," ISS program manager Mike Suffredini said.
"There was never any risk to the crew," he added.
Earlier, NASA's Jim Kelly at mission control in Houston said a review of the data appeared to show a sensor problem or a computer relay issue could have led to the alarm.
"It is becoming a stronger case that this is a false indication, which is great news," Kelly said to US astronaut and space station commander Barry Wilmore in an exchange broadcast on NASA television.
The one European and two American astronauts taking shelter in the Russian segment returned to the US side at 3.05pm, NASA said.
A sample taken of the air on the US side of the station by American astronaut Terry Virts and Italian astronaut Samantha Cristoforetti showed "no ammonia indication", NASA said on Twitter.
Others on board the station include Russian cosmonauts Elena Serova, Alexander Samokutyaev, and Anton Shkaplerov.
The problem became apparent when flight controllers in Houston "saw an increase in pressure in the station's water loop for thermal control system B, then later saw a cabin pressure increase that could be indicative of an ammonia leak in the worst case scenario", NASA said on its website.
The Expedition 42 crew had been awake for about two hours before the alarm sounded, and was at work unloading the SpaceX Dragon cargo carrier which arrived days ago with more than 2.5 tons of supplies and science experiments.
Suffredini said some freezers had to be temporarily turned off, but that no research had been lost.
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