The presidential panel investigating the Challenger shuttle disaster has found new evidence that NASA managers at Marshall Space Flight Center overrode internal warnings that joints in the space shuttle's solid-rocket boosters might fail and that one official repeatedly signed waivers allowing the craft to keep flying despite the flaws, according to a source close to the commission.

"The commission has concluded," the source said, "that this process was as flawed" as that of the decision-making process that took place the night before the launch of the Challenger on Jan. 28. The craft blew apart, killing its crew of seven.

In an eight-hour closed session yesterday, which was said to be their final hearing, members of the commission once again interviewed officials from Marshall Center in Huntsville, Ala., which handles the shuttle's propulsion, and from contractor Morton Thiokol Inc., which makes the rocket boosters, about the history of problems with the booster joints dating as far back as 1978.

The new evidence involved a series of agency decisions beginning in July 1985. Because of concern about the joints' performance, NASA officials at Marshall imposed a restriction known as a "launch constraint," which meant no launch was to take place until the problem was fixed, the source said, confirming a CBS News report.

Before each of the six shuttle flights after that date, the source said, project director Lawrence Mulloy signed waivers that allowed the shuttle to be launched, "even though the problem -- namely erosion on the O-rings -- was not resolved," the source said.

Mulloy was also one of the Marshall officials who challenged objections voiced by Thiokol engineers the night before the launch. The engineers warned that cold temperatures at the Florida launch site might prevent the O-ring seals on the booster joints from working.

Top NASA officials have testified in commission hearings that they were never informed of the Thiokol engineers' warnings.

Investigators now believe that the combination of booster joint design factors and cold temperatures caused the destruction of the space shuttle.

The launch constraint procedure is used by NASA when a concern is raised about hardware performance, the source said. It is separate from the formal list defining which hardware is critical to flight safety.

The fact that waivers were signed, in this case, does not mean that Marshall officials were ignoring the problem with the joints, he added, but that they decided not to wait until they had fixed it. NASA officials could not be reached for comment.

NASA engineers at Marshall had expressed concern about the design of the booster joints as early as 1977. And a memo in 1979 labeled the joints "completely unacceptable," the Orlando Sentinel reported last month. Alarm about the joints had become even more acute in the summer of 1985, according to previous testimony before the commission, with one Thiokol engineer suggesting in a memo that the problems could lead to a "catastrophe."

The commission's final report is due out in early June. It is expected to include not only conclusions on the cause of the accident but also recommendations for changes at the space agency, such as appointment of a special safety panel.

Meanwhile, George B. Hardy, a NASA official who played a key role in the decision to launch the shuttle, has retired, continuing a management shakeup within the agency.

Hardy, 55, was deputy director of science and engineering at the Marshall center. On the evening of Jan. 27, according to testimony before the commission, Hardy told Thiokol engineers he was "appalled" at their recommendation to delay the launch.

Marshall spokesman Ed Medal said Hardy had planned to retire for some time and, as far as he knew, the decision was unrelated to the shuttle investigation. Two other top National Aeronautics and Space Administration officials involved in the decision to launch the Challenger -- Stanley Reinhartz, the manager of shuttle projects at Marshall, and Jesse Moore, the former associate administrator for space flight -- have had their duties reshuffled since the accident, although Moore's change in position had been announced before the Jan. 28 launch.

In the late 1970s, Hardy was the manager of the solid-rocket booster project at Marshall when agency engineers first complained about the design of the rocket joint that has been blamed for the Challenger accident.

He was one of several Marshall officials who argued with Thiokol engineers Jan. 27 when they objected to launching the Challenger the next morning in cold weather. Hardy was quoted by Thiokol engineers as saying that he was "appalled" by their recommendation not to launch -- a comment the engineers construed as pressure to change their position. Hardy testified that he was appalled not at the recommendation but at engineering data used by Thiokol.