UPDATE | Astronauts flee US side of space station but no sign of leak

CAPE CANAVERAL, Fla. (AP) — In a rare scare, astronauts fled the American side of the International Space Station today after an alarm indicated a possible toxic leak. NASA later said a computer problem likely set off the alarm, rather than escaping ammonia coolant.

"No signs of a leak," NASA reported via Twitter.

In the meantime, the six crew members huddled safely on the Russian side of the orbiting outpost, as Mission Control analyzed the data. As it turns out, the astronauts had to rush over to the Russian segment twice: the first time when the alarm sounded, the second after an initial all-clear.

"Hey everybody, thanks for your concern," Italian astronaut Samantha Cristoforetti said in a tweet. "We're all safe & doing well in the Russian segment."

The "unscheduled excitement," as NASA called it, occurred about 4 a.m. EST, well into the station crew's workday.

As alarms blared, the astronauts followed emergency procedures in slapping on oxygen masks, taking cover in the Russian quarters, then sealing the hatches between the U.S. and Russian sides. At the same time, flight controllers at NASA's Johnson Space Center in Houston turned off nonessential equipment.