NASA’s segment of the International Space Station (ISS) was temporarily evacuated yesterday morning in response to a false alarm that led the astronauts to believe that ammonia had leaked into the spacecraft’s air supply.

Just a few hours into Wednesday’s workday at about 3 a.m., the ISS U.S. segment’s sensors sounded and the team of researchers sought refuge in the Russian compartment.

The crew, which includes engineers Terry Virts, Samantha Cristoforetti and Station commander Barry Wilmore, has since safely returned to their cabin and expects to resume work on Thursday.

NASA believes that the alarm was “caused by a transient error message in one of the station’s computer relay systems, called a multiplexer-demuliplexer,” according to an official statement from the agency. However, the investigation is still ongoing.

Liquid ammonia is used as a coolant aboard the orbiting ISS, particularly with pipes and heating apparatuses.

“We saw an increase in water loop pressure, on the crew cabin pressure, that could be indicative of an ammonia leak in a worst-case scenario,” NASA’s Kelly Humphries told The Washington Post.

Upon their return to the U.S. segment, the crew was instructed to wear protective masks. Following a series of tests ensuring the air was clear, they removed their masks.

Station program manager Mike Suffredini assured Reuters that “There was never any risk to the crew.”

Wilmore and his colleagues now have the green light to continue their vast slate of experiments in the coming days.

The ISS is maintained by Europe, Japan, Canada, the United States and Russia. NASA admits that the space station is “the most politically complex space exploration program ever undertaken.”

Since its launch in late 2000, the station has orbited the Earth 57,361 times.