NASA's first shuttle flight in more than two years has been put off indefinitely as the space agency mounts a massive investigation into why a fuel gauge failed right before Discovery's scheduled liftoff.

Engineers were no closer Friday to knowing why the gauge acted up two days earlier.

"We are going forward on a day-by-day basis," said deputy shuttle program manager Wayne Hale. "We have got the entire resources of the agency behind us to troubleshoot this problem."

Hale said that once the problem was identified and fixed, it would be another four days before the shuttle could launch.

"Everybody is going to want to ask, 'What is that date going to be?' Well, I don't know," he said.

It was the latest setback in NASA's grueling and drawn-out quest to return to space and recover from the 2003 Columbia tragedy (search). The space agency has made a multitude of safety improvements to the aging shuttle to avoid future catastrophes, efforts that have repeatedly delayed Discovery's mission.

Engineers are looking at whether any of those safety improvements — like additional heaters on the external fuel tank to prevent dangerous ice buildup — may be contributing to the failure of one of the four fuel gauges in the tank. When the gauge showed an improper setting, Wednesday's launch was canceled.

Hale said it's possible NASA (search) could try to launch again late next week, "but that would require a very near-term lucky find" of the source of the problem.

Discovery's seven astronauts opted to remain in Cape Canaveral (search) and wait it out, rather than return to their homes in Houston.

Managers had held out hope, however slim, that they might be able to launch Discovery within a few days. But with engineers no closer to figuring out why the fuel sensor malfunctioned Wednesday — a potentially deadly problem — NASA had no choice but to call for a lengthy standdown.

NASA is up against the clock. If extensive repairs are needed and the shuttle has to be moved off the launch pad and into the hangar, the flight could end up being bumped into September to ensure a daylight liftoff.

The space agency wants a clear view of the ascending shuttle in order to spot any launch damage. When combined with the constantly changing location of the international space station, Discovery's destination, this means that the shuttle must fly by the end of July or remain grounded until Sept. 9.

"We are not in any sense of the word becoming pessimistic about making the July launch window," Hale stressed to reporters. "We are here for the duration. We are committed to giving this the good, old college try until we get the problem resolved."

For the second full day, 12 teams of engineers around the country pored through data for clues as to why the hydrogen-fuel sensor failed during Wednesday's routine pre-launch test.

It was the same type of problem that marred a fueling test of Discovery in April. NASA replaced an electronic box and cables associated with the fuel gauge, as well as the fuel tank itself for other reasons, and chalked the failure up as an "unexplained anomaly."

This time, NASA is considering every angle, including the effects of a fully fueled spacecraft with all its systems running, said John Muratore, shuttle systems engineering manager.

Hale also suggested for the first time that if the problem can't be explained after days or weeks of effort, NASA might be forced to consider flying with the fuel gauge mystery unsolved.

The fuel gauges are needed to make sure a shuttle's main engines don't run too long or not long enough on the climb to orbit. Either case could prove deadly.

NASA's three surviving shuttles have been grounded ever since Columbia shattered in the sky over Texas on Feb. 1, 2003, killing all seven astronauts. A chunk of fuel-tank foam insulation came loose during liftoff and pierced the left wing, sending the spacecraft on a deadly descent two weeks later.

Watch "Discovery: Return to Space," a one-hour special on the FOX News Channel, on Saturday at 9 p.m. and 12 a.m. EDT.