Nearly a week after a faulty fuel-gauge reading halted the first shuttle countdown since the Columbia tragedy (search), NASA had yet to uncover any solid clues to the problem Monday and put off the flight of Discovery until at least next week.

"Right now, I can tell you that we're still looking for the problem," shuttle program manager Bill Parsons (search) said at an evening news conference. "We've waited two-plus years, 21/2 years to be here. We're trying awfully hard to resolve this issue."

Added his deputy, Wayne Hale (search): "A few days more when it's all said and done, to make sure we're flying safely, is not a problem in the bigger scheme of things."

Discovery — indefinitely grounded since Wednesday's fuel gauge failure — will fly no earlier than July 26, Hale said.

NASA may decide to conduct yet another fueling test on the shuttle that day or soon afterward, which would bump the liftoff another one or more days.

"Hopefully in the next 24 to 48 hours, we will find the glitch that has got us all confused or frustrated or pick your adjective, and be able to fix it and go forward," Hale said. "But I think Tuesday is probably the earliest day that we would be looking for a launch, even in that optimistic case."

One of four hydrogen fuel gauges at the bottom of Discovery's external fuel tank failed during a prelaunch test Wednesday, forcing NASA to delay the first space shuttle mission since Columbia's catastrophic re-entry in 2003. The postponement came just two hours before the scheduled liftoff; the seven-member crew was already on board.

Since then, technicians have crawled around inside Discovery's engine compartment and checked for anything that might explain why the fuel gauge malfunctioned, and engineers have conducted a battery of tests. As of Monday evening, no one had found anything amiss or even suspicious.

Even more aggravating, NASA has been unable to duplicate Wednesday's failure, which could prove disastrous in flight. The fuel gauges are intended to keep a shuttle's main engines from shutting down too early or too late after liftoff.

If NASA does not launch Discovery in the next two weeks, then the spacecraft will remain grounded until September.

The launch window officially extends until the end of July to allow for a daylight liftoff; the space agency wants good camera views all the way through tank separation eight minutes into the flight. But managers are considering stretching that into August by a few days, even though the lighting would not be as good when the empty fuel tank is jettisoned.

The next launch window with suitable lighting would open Sept. 9.

Columbia was brought down by a broken section of fuel-tank foam insulation that smacked into the left wing at liftoff. The shuttle shattered in the sky over Texas on Feb. 1, 2003, while returning from space. All seven astronauts were killed.