Nasa is investigating the possibility of a gaseous oxygen leak, posing a serious fire risk, during the launch of the space shuttle Discovery in July.
Shuttle crew tested in-flight repairs and delivered supplies to the ISS
Experts are studying data from the test flight - the first since space shuttle Columbia broke up over Texas in 2003.
Engineers uncovered possible evidence of high concentrations of the gas in the rear engine compartment about two minutes after lift-off.
A leak could lead to a fire or even an explosion in flight.
Officials said the issue must be resolved before the next shuttle mission, which Nasa hopes to launch in May.
"We're going to err on the side of caution," Kyle Herring, a spokesman for the US space agency's (Nasa) Johnson Space Center in Houston, told the newspaper Florida Today.
"We're going to run this thing to ground and make sure we understand it."
Strict Nasa rules call for a launch attempt to be scrubbed if gaseous oxygen concentrations in the compartment reach 500 parts per million.
The indications come from the six "catch bottles" that gather samples of air within the engine compartment during flight.
Data from three of the devices showed gaseous oxygen levels during flight were normal. Data from a fourth was corrupt, officials said.
But two of the catch bottles indicated levels of gaseous oxygen in the compartment about two minutes into flight were higher than Nasa allows.
Foam loss problem
However, no indications of a leak were evident in data on the shuttle's performance during its nine-minute climb into orbit on 26 July.
A serious leak would probably have left the shuttle short of its target. This would have required an extra firing of Discovery's twin orbital manoeuvering engines to boost it the rest of the way.
Last week, a Nasa spokesman said it might consider flying the next shuttle mission without the protective foam ramp that broke away from Discovery's external tank during its July launch.
A one-pound chunk of foam was shown tearing away from the external fuel tank's Protuberance Air Load (Pal) ramp during lift-off.
The ramps were installed on the external tank early in the programme's history to shield external pressurisation lines and a cable tray from aerodynamic buffeting.