"We are still looking for the problem," shuttle program manager Bill Parsons said.

At the soonest, NASA said, Discovery could blast off on July 26.

"We will try to make that window," which closes July 31 for Discovery's intended rendezvous with the International Space Station.

NASA is well aware that this is the first launch attempt since Columbia tore apart in February 2003, killing all seven aboard.

"Our number-one goal is to find this problem and fix this problem," said Wayne Hale, deputy shuttle program manager, during a press conference at the Kennedy Space Center near Cape Canaveral in Florida.

After NASA scrubbed Wednesday's lift-off, officials initially indicated they might be able to send Discovery into orbit on Saturday, then said a lift-off could not take place before the end of next week, and finally said they could not set a date at all.

Engineers at the Florida seashore installation Friday emptied the shuttle's massive external tank and unloaded cryogenic reactants that power fuel cells, to allow safe access to an electronics box that processes signals from the tank's fuel level sensors.

If the shuttle is not ready for a July lift-off, NASA will have to wait for the next launch window, September 9 to 24.

The delay is a major disappointment for the agency seeking to recover its prestige after Columbia broke up as it re-entered Earth's atmosphere on February 1, 2003.

The agency was forced to rethink the shuttle program and has made changes to the shuttles to reduce the risk of a new accident.

The new delays were compounded by NASA's desire to launch during daylight hours. That reduces the window during which Discovery can lift off for its rendezvous with the International Space Station.

Hundreds of engineers were mobilized to study as many as 200 potential causes for the malfunctioning fuel gauge system, and how to fix it.

If the problem is serious, NASA will have to roll the shuttle back five kilometers (three miles) from its seashore pad to its hangar.

The faulty sensor is one of four that sends data on the levels of hydrogen in the spacecraft's massive external fuel tank, to determine when the three engines should be shut off during the ascent into orbit.

NASA had a similar problem in April but never found the cause. The problem disappeared after technicians replaced cables and electronic boxes attached to the gauges.

In another glitch, a plastic and foam protective cover fell 20 meters (60 feet) off one of Discovery's windows, damaging tiles on one of the two orbital maneuvering systems on Tuesday.

The US space agency had initially hoped to launch Discovery in May, but eventually decided it needed more time to prepare the orbiter and to conduct a closer review of pre-flight preparations.

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