NASA's investigation of a major fuel leak that forced the shuttle Columbia to be rolled back to its hangar yesterday has raised questions about seven other leak-prone fuel lines that were approved for flight, including two that flew without incident and one installed in the orbiter Atlantis for a July launch, sources said.

The concern centers on a critical 17-inch "quick disconnect" pipeline that carries explosive, supercold liquid hydrogen from the giant external fuel tank into the belly of the orbiter. The system is considered one of the most dangerous on the shuttle and was redesigned for added safety following the 1986 Challenger disaster.

Dan Germany, orbiter projects manager at Johnson Space Center in Houston, confirmed that engineers tracing the history of Columbia's leaky part discovered that it and seven others like it had leaked during tests required before the parts were approved for flight. It is possible that modifications may have to be made to components still awaiting flight, and that the testing procedures may have to be changed, he said, but "it's too early to draw a conclusion."

The apparent problem was discovered Friday, but officials decided anyway to "mate" Atlantis with its fuel tank for a July 13 launch. They did this, Germany said, because its fuel line passed all recent leak tests and because the problem may show up only under "full cryogenic flow," that is, when liquid hydrogen, which has a temperature 423 degrees below zero Fahrenheit, is flowing through the lines when the shuttle is fueled shortly before launch. The preliminary tests are done with liquid nitrogen which is about 100 degrees warmer.

"When we tank, that'll be the real proof of the pudding," according to Germany. He said extensive procedures are in place to detect any hydrogen leak that may develop at the pad. The same system detected hydrogen gas flooding Columbia's aft engine compartment, delaying its launch from late May to August at the earliest.

Puzzled engineers have so far been unable to find the Columbia leak, which is not behaving like previous leaks and may be a problem in seals or other parts in the line on either the orbiter or the tank side.

Despite extensive testing before launch, technicians failed to find at least one major leak -- the one aboard Columbia.

Suspect "disconnects" were also flown on two shuttle flights that launched a NASA communications satellite and a secret military payload in the winter and summer of 1989. An unusual vapor cloud or frost was noticed around the disconnect just before launch on one of them, officials said, sign of a possible tiny leak.

What the eight umbilical-like fuel lines have in common occurred during testing at the manufacturer, Parker Hannifin, in Irvine, Calif., officials said.

The system consists of two halves, a pipe and valve connected to the orbiter, which is flown repeatedly, plus a similar pipe and valve that is connected to the external tank, which is jettisoned after launch and replaced for each launch. The tank portions were tested in Irvine, using an orbiter "slave unit" to simulate the reusable orbiter half, Germany said.

After each of the eight leaked, the orbiter unit was replaced with a flat sealer plate and when the tank half was tested alone, there was no leak. Engineers with various contractors and at several levels of NASA management accepted the explanation that the leak was in the ground test unit, and not in the tank components that were to be flown.

Because the leaks may not show up until the device is chilled to the temperature of liquid hydrogen, this raises the possibility of repeated major disruptions in the shuttle launch schedule and could mean that there are other shuttle components whose testing will also have to be modified.

"It could be that we're learning something here now," he said. "We'll just have to play these cards out and see what they tell us in the end."