Lockheed Martin Corp. has asked investigators to reopen the portion of its Michoud Assembly Facility where insulating foam is applied to the external tanks used to power space shuttles, so that work on the tanks can resume.

Damage from foam that broke off an auxiliary fuel tank and hit the shuttle Columbia's left wing is considered one of the possible culprits in the loss of the shuttle Feb. 1, though investigators say they are looking at many other possibilities.

Hours after Columbia disintegrated over Texas, more than 20 investigators descended on the Michoud facility here, closing several parts of the 438-acre campus, including areas used to clean and prime the tank. While the rest of the plant has been reopened, the foam-application rooms remain closed.

"Only a small area of foam application has been in question, and we would like to proceed with all of the additional work that still needs to be done," said Lockheed spokesman Marion LaNasa.

"We would like to get back to full production. We have employees who have not been able to get into that work area for a month, and we think it is important to get back to work," he said, noting that it could take two to six months for the investigative panel to reach a conclusion. "We need to keep our skilled workforce intact, and we need to keep them working in order for them to maintain those skills they developed over many years." About 35 Michoud employees are certified to apply foam, and the average employee has 20 years' experience.

The investigative panel has not responded to Lockheed's request, LaNasa said.

Fifteen tanks are in different stages of production, and three of those are ready to undergo the three- to four-month foam-application process, according to company officials. Five completed tanks are stored on the Michoud campus, including one that is a near-twin of Columbia's, and all have been impounded by the investigative team. Another three, assigned to specific upcoming missions, have already been sent by barge to the Kennedy Space Center in Florida.

NASA investigators are preparing to test the "sister" tank, completed around the same time as Columbia's. But first they will use a "cousin" tank completed a year before, said Laura Brown, a spokeswoman for the investigative panel.

The cousin is a "super-lightweight" tank, unlike the heavier "lightweight" tank used on Columbia, but it has a similar bipod ramp and foam, Brown said. Testing will allow investigators to try out their tools and techniques before dissecting the sister tank, she said. "They are going to do some mock-up testing first," Brown said. "After we get the results of that, then we use foam dissection to look inside and see if there are any manufacturing [problems] or if they see anything unusual."

The New York Times reported today that investigators are looking into whether workers at the plant used an ineffective adhesive to attach a secondary layer of insulation, called "ablator," to Columbia's tank. According to the report, workers may have been misled by a handwritten label on a container that misstated the deadline by which the adhesive had to be applied.

Brown said today that she was unaware of such an inquiry but that it could be going on. Gen. Duane Deal, a member of the investigative panel, will visit the plant Monday, she said. "This thing about the ablator is one of the things he will ask about, but it's not why he's going out there," Brown said.

Added David Youngman, a NASA spokesman: "We know the investigators are looking at every aspect, whether it be the glue or the covering. We don't know of any particular reason that they're looking at it other than they're looking at all possibilities."

Even as Bethesda-based Lockheed attempts to get its plant to full production, changes to the tanks could be imminent. When Columbia launched in mid-January, NASA engineers were already studying how to improve the way the foam is applied and whether a secondary layer of insulation used under it could be eliminated, said Neil Otte, deputy manager of the external tank project at NASA's Marshall Space Flight Center.

"Obviously we had 60 flights in which we had absolutely no problem with that configuration," Otte said. But he added that NASA would do away with the secondary layer if it could.

The agency was also considering ways to make the application of foam more consistent. While a machine applies most of the foam, employees manually apply it to certain areas of the tank, including the bipod ramp.

"We have guys out there spraying the foam on, and so we try to give them the environment in which they can do the same thing over and over again," Otte said.

Engineers had not made specific recommendations and were still studying how those changes could be accomplished when Columbia launched, Otte said. They never considered delaying the Columbia mission to allow changes to be made first, he said.

"The program looked at it very carefully and once again came to the same conclusion that they have come to from Day One of the program: It was an acceptable risk," Otte said.

Foam debris "was never considered a safety-of-flight issue. It is an orbiter-maintenance issue," he said. "We don't want to see foam loss, and we do everything we can to minimize it."

More than 500 employees at Lockheed Martin's Michoud Assembly Facility pose in front of shuttle tanks in 1990.