The crew cabin of the Challenger broke apart from the rest of the space shuttle and tumbled out of the initial fireball relatively intact but then exploded independently about 1 1/2 seconds later, a source close to the presidential commission investigating the accident said yesterday.

The finding, which emerged early in the investigation but has received little public notice, confirms the observations of a number of amateurs who made their own studies of videotapes of the explosion.

The tapes are fuzzy but appear to show that as the fireball was reaching its maximum size, an object with markings that resemble those of the cabin emerged from one side and, just as it cleared the fireball, exploded by itself.

Investigators believe that explosion was caused by the rupture of tanks containing more than 2,200 pounds of propellants stored in the orbiter's nose, which is just ahead of the cabin. The propellants, a fuel and an oxidizer that ignite spontaneously when they mix, are used to maneuver the shuttle after it is in orbit.

Now that the cabin's wreckage is being retrieved from the floor of the Atlantic, along with the remains of the crew, the existence of the second explosion raises new questions about the sequence of events that killed the astronauts.

One of the amateurs, David Balaban, a video editor in the motion picture department of Walter Reed Army Institute of Research, raised the possibility that if the cabin was nearly intact when it flew out of the fireball, it might have been possible for the astronauts to survive if they had ejection seats that worked before the propellants exploded. There are no ejection seats in the shuttle.

Even if there were, the commission sources said, it is believed that the astronauts were probably already dead, killed by the force of the original explosion. William Brockman of Oak Creek, Wis., is another of the amateurs who analyzed the tapes.

Meanwhile yesterday, the commission met privately to review new film of the early moments of the Challenger's Jan. 28 flight. Sources said the films confirmed the scenario that has been developing for some weeks; that the cold-hardened O-ring seals in one of the solid rocket boosters failed in the first second of launch, leading less than a minute later to a burn-through of the booster wall that eventually tore the booster loose, rupturing the huge external fuel tank. It was the explosion of this fuel that caused the initial fireball.

The commision also issued a statement that it would appoint a panel of experts to test the O-rings' performance when chilled to the sub-freezing temperature in which Challenger launched. The panel discounted earlier tests that were said to exonerate cold as a factor because the tests were done by the same officials who had disregarded warnings not to launch in cold weather.

The panel, which will report to the commission, is to include experts from the National Transportation Safety Board, TRW Inc., and the Air Force, the source said.