1625 GMT (11:25 a.m. EST)
NASA's space station program manager says ground teams do not believe any ammonia leaked on the complex this morning, and attention is focusing on a computer glitch that gave mission control errant readings.

"At this point, the team does not believe we leaked ammonia," said Mike Suffredini, NASA's space station program manager. "What we are dealing with is this failure of probably a card inside one of the multiplexer-demultiplexer. It's just a computer that sends telemetry down and brings commands back up. This card has a number of measurements on it, and those are the measurements we lost."

Suffredini said mission control received several indications of an ammonia leak, including rising levels in an accumulator sensor and an increase in atmospheric pressure inside the space station's cabin.

"Those two cues together had the team go into a conservative mode to go and protect the crew," he said.

Mission control told commander Butch Wilmore and flight engineers Terry Virts and Samantha Cristoforetti to evacuate the U.S. modules and head into the space station's Russian segment and close hatches dividing the two sections of the outpost.

Measurements in the Russian segment show no indication of any ammonia in the space station's atmosphere, he said.

Mission control powered down the one of the space station's two ammonia coolant loops after the alarm around 4 a.m. EST (0900 GMT) this morning. The suspect coolant loop -- coolant loop B -- is being reactivated along with systems in the U.S. segment, Suffredini said.

"Now we have this big road in front of us to get back reconfigured," Suffredini said.

Once all the U.S. systems are back online, engineers will look at telemetry to determine data still indicates a possible leakage of ammonia. If there is an all clear, Wilmore, Virts and Cristoforetti could be cleared to go back to the U.S. segment of the station tonight.

"We would like to get the crew back into the U.S. segment tonight," he said. "That would be our goal. While it's inconvenient for the crew to the be in the Russian segment, it's certainly not unhealthy for them."

1325 GMT (8:25 a.m. EST)
Astronaut Jim Kelly, serving as spacecraft communicator in mission control in Houston, just told the space station crew that it looks like the data that triggered concern over a possible ammonia leak this morning was a false alarm.

"It's becoming a stronger case that this is a false indication, which is great news," Kelly radioed the space station.

"Outstanding news," replied commander Butch Wilmore. "Great news, guys, and we'll be ready to do whatever you need us to do when the time comes."

Kelly said mission control is looking at whether the crew can re-enter the U.S. segment of the space station later today. The U.S. modules were blocked off when the crew evacuated into the Russian segment and closed hatches between the sections.

Telemetry data from the space station early this morning, U.S. time, indicated an increase in pressure in the water loop of the space station's coolant loop B. The cooling system uses water to collect heat from inside the space station's modules and dissipates the heat by routing ammonia through radiators outside the complex.

The concern was that ammonia, which is designed to stay in the station's external loop, may have leaked into the water loop that runs inside the lab's pressurized modules.

But no hard data indicated an actual leak, said NASA TV commentator Rob Navias.

Engineers are now looking at whether the alarm may have been caused by a faulty sensor or computer relay.

1302 GMT (8:02 a.m. EST)
Mission control detected an increase in pressure in a water line in one of the International Space Station's two coolant loops around 4 a.m. EST (0900 GMT), according to NASA TV commentator Rob Navias.

As a precaution, commander Butch Wilmore and flight engineers Terry Virts and Samantha Cristoforetti were told to move to the Russian segment of the space station and close hatches dividing the U.S. and Russian sections of the complex, Navias said.

1230 GMT (7:30 a.m. EST)
Two NASA astronauts and an Italian flight engineer are sheltering in the space station's Russian segment as mission control analyzes a possible leak of toxic ammonia coolant aboard the complex.

NASA says there is no confirmation of a leak, but they ordered commander Butch Wilmore and flight engineers Terry Virts and Samantha Cristoforetti into the Russian part of the outpost as a precaution.

The crew is safe, according to reports by NASA and the Russian space agency -- Roscosmos.