Electrical failures in the power supply to a fuel-line heating unit caused engineers to scrub the what would have been the second-to-last shuttle flight ever just hours before liftoff on Friday.

A plan to try the launch again on Monday was scratched once engineers realized the problem could take more time to fix.

"We still have a lot of work and a lot of offroads that could take us one way or another," Mike Moses, NASA launch integration manager, told reporters on Sunday.

The launch will be "not any earlier than the 8th," and possibly even later, he said.

"Just take that as a target for when we are going to start talking again about another launch date."

Endeavour is poised to carry a potent, multibillion dollar tool for searching the universe in the penultimate flight for NASA's 30-year program.

After Endeavour returns from its 14-day mission and Atlantis launches for a final time in June, the iconic space shuttle program will close for good.

That will leave Russia as the sole taxi for astronauts heading to and from the orbiting space lab until a new spaceship is built by a partnership between NASA and private companies, by 2015 at the earliest.

Officials blamed electrical problems in a unit that would have prevented fuel from freezing in orbit for last week's scrubbed launch.

"Technicians and engineers are narrowing in on the likely source of what caused heaters on a fuel line for space shuttle Endeavour's auxiliary power unit-1 (APU-1) to fail on Friday," NASA said.

"Teams will meet Monday and are expected to determine a new 'no earlier than' next launch attempt for Endeavour at that time," the US space agency said in a statement.

The crew of six astronauts -- five Americans and one Italian -- was headed back to Johnson Space Center in Houston "for a few days of additional training" before returning for the next launch attempt, NASA said.

Wounded lawmaker Gabrielle Giffords, who flew to Florida from a rehabilitation facility in Texas to see the launch, also returned to the hospital that is treating her in Houston but will attend the next attempt, her office said.

Giffords was shot in the head at a neighborhood political gathering in January but was allowed by her doctors to take a break from rehab in order to watch her husband, Mark Kelly, command the shuttle.

"Team Giffords had an easy flight back to Houston today," said a post on the Arizona lawmaker's official Facebook page.

"Gabby will have a couple days of good therapy... Then back to FL for the launch!" it said.

The shuttle Endeavour is the youngest of the three-member space flying fleet. It was built in the wake of the Challenger disaster in 1986 and flew its first mission to space in 1991.

Discovery, the oldest, flew its last mission in February and March, and is in the process of being stripped of all its valuable components ahead of its retirement in a museum on the edge of the US capital Washington.

Endeavour will carry a $2 billion, seven-ton particle physics detector, the Alpha Magnetic Spectrometer-2, which will be left at the space station to scour the universe for clues as to how it all began.

The final shuttle mission, Atlantis' 12-day voyage to the ISS set for late June into early July, will bring the storied program to its final end.

There have been six space shuttles in all, including Endeavour, Atlantis and Discovery.

Enterprise was a prototype that never flew in space; Challenger exploded after liftoff in 1986, killing all seven on board; and Columbia disintegrated on its return to Earth in 2003, also killing seven astronauts.

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