CAPE CANAVERAL, Fla. -- Nearly a week after a faulty fuel-gauge reading halted the first shuttle countdown since the Columbia tragedy, NASA had yet to uncover any 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 said at an evening news conference. "We've waited two-plus years, 2 1/2 years to be here. We're trying awfully hard to resolve this issue."

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Added his deputy, Wayne Hale: "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."

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Discovery - indefinitely grounded since Wednesday's fuel gauge failure - will fly no earlier than next Tuesday, Hale said.

NASA may decide to conduct yet another fueling test on the shuttle that day or soon afterward, with the external tank fully loaded with propellants. Such a test could push the liftoff even further into next week.

Hale said one big question being debated is if the problem does not recur in a fueling test, and the troublesome fuel gauge works properly, "is that good enough to go fly the next day?" - or even, possibly, the very same day.

"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."

If Discovery does not fly in the next two weeks, then that's it until September.

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 over Texas 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 night, no one had found anything amiss or even suspicious. An electronics box associated with the fuel gauges passed inspection, and the cables and wiring looked good. NASA hopes to complete this initial round of testing by Wednesday.

Even more aggravating, NASA has been unable to duplicate last Wednesday's failure, which could prove disastrous in flight. Barring any "eureka moments" as Hale calls them, the next step would be to fill Discovery's tank with super-cold fuel in a test, to see if the problem recurs.

The fuel gauges are intended to keep a shuttle's main engines from shutting down too early or too late after liftoff, both potentially dangerous situations. Only two of the four are needed to ensure safety, but NASA requires all four to be operating.

Until the 1986 Challenger launch explosion, NASA required only three of the four fuel gauges to be working. Mission managers are considering going back to that earlier looser rule, although they are still hoping to solve the problem, Hale said. NASA's 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 to check for any damage. 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.

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