Cape Canavarel, Fla. ? NASA has indefinitely put off its long-awaited return to space, saying Friday that engineers were no closer to knowing why a fuel gauge acted up right before a scheduled liftoff 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.”

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

“Everybody is going to want to ask, ‘What is that date going to be?’ Well, I don’t know,” Hale said.

It was the latest setback in NASA’s grueling and drawn-out quest to return to space and recover from the Columbia tragedy 2 1/2 years ago. 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 malfunctioned, Wednesday’s launch was canceled.

Hale said it’s possible NASA 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 and just wait it out, rather than return to their homes in Houston.