William P. Rogers, chairman of the presidential commission investigating the Jan. 28 explosion of the shuttle Challenger, said yesterday that the panel is focusing on the relationship between the unusually cold weather before the launch, and problems with the solid rocket booster and external fuel tank of the vehicle.

Another panel member, Nobel laureate physicist Richard P. Feynman, said that based on information received so far, a failure of the O-ring seals on a booster assembly joint "looks like the most likely cause at this point. . . . If you asked me now, I might say that is it."

Feynman, as well as Rogers, stressed that the investigation is far from complete.

The comments, made after commission members completed a tour and closed-door briefing at the Kennedy Space Center in Florida, were the first official confirmation of the direction of the investigation is moving.

The commission also yesterday released a dramatic "time line" chronology of the split-second chain of events that destroyed the shuttle and killed its crew of seven.

In another development, Aviation Week & Space Technology, the authoritative industry magazine, reported that investigators are probing the possibility that supercooled fuel may have leaked from the huge external tank next to the booster rocket while the shuttle was on the launch pad, freezing the O-rings so that they could not seal the rocket.

The magazine said temperature measuring devices recorded parts of the right-hand booster at 7 degrees Fahrenheit before the flight, considerably lower than the safety specifications for the rocket. It said ground crews did not inform responsible officials of the measurements.

The time line narrative, compiled after the accident, begins with the notation of "O.O -- SRB solid rocket booster ignition command, nominal."

Five-hundredths of a second later, the shuttle system lurches to life: "First movement," noted the NASA chroniclers.

At .445 seconds: "First evidence of smoke." The narrative says this is the earliest view of the unexplained puff of unusual black smoke that burst from the side of the rocket just after ignition.

One prominent solid rocket scientist thinks that this smoke may show the destruction of the seals at the right booster's aft main joint, when white-hot exhaust gases broke through and burned the two O-rings designed to prevent leakage.

The black smoke is "seen to extend half-way across the right SRB by 2.147 seconds, and. . . continues through tower clear, roll maneuver and as late as 12-13 seconds into the flight."

For 45 seconds, all seems well -- "nominal," as the rocketeers call a perfect flight.

Then, the trouble reappears: "Approximately 58.77 seconds, there is an indication of smoke from the right SRB forward of the aft attach ring. A well-defined, intense plume then appears.

"After 60 seconds, telemetry data appears to show a small divergence in chamber pressure between right and left SRBs. A camera shows a glow on the right SRB after 66 seconds, apparently merging with the plume at about 67.65 seconds.

"At 73.175 seconds," asserts the narrative, "a sudden cloud is seen alongside the external tank, and a flash from the region between the orbiter and the external tank liquid hydrogen tank is seen a fraction of a second later, followed by an explosion near the SRB's forward attach point.

"Telemetry shows the effect of the explosion, with the last data received at 73.621" seconds.

Jim Mizell, a retired NASA engineer who is a briefer at the Kennedy Space Center, said the establishment of the time line from thousands of bits of data gives NASA the opportunity to recreate conditions of each event in the catastrophe in a way that will shed light on what happened.

Engineers and analysts can use this data to experiment with the SRBs, Mizell said. They can try to duplicate the blast conditions, either by computer simulation or actual field tests. "This paints a specific picture of how the Challenger was destroyed. This is the evidence of the trajectory that is necessary to find out what happened to the shuttle."

According to the lead article of the Feb. 17 edition of Aviation Week, NASA has found evidence indicating that temperature readings of 7 degrees and 9 degrees F. were recorded in two places on the lower portion of the right SRB just 90 minutes before the fatal launch.

The magazine said the temperatures were taken by a special team sent to investigate ice conditions on the launch pad. A cold snap had sent the mercury at Pad 39B into the 20s, and it was 38 degrees F. when the shuttle lifted off at 11:38 a.m. on Jan. 28.

Concerned about the low temperatures, a special ice team with infrared measuring instruments surveyed the pad, but did notify superiors of their readings. The magazine said NASA is now calibrating the instruments to make sure they are accurate.

If the readings turn out to have been correct, the steel casing of the booster was far colder than the surrounding air. This would indicate that some unusual source of cold had impinged on the booster, which in turn points to the idea of a possible pinhole opening in the massive external tank which supplies the orbiter's main engines with fuel.

Liquid hydrogen has a temperature of -423 degrees F., and liquid oxygen, -297 degrees F. Several experienced engineers at Cape Canaveral discounted the theory, saying pressure sensors inside the external tank would detect such a leak.

They also said that a plume of supercold hydrogen or oxygen would be easily visible.

No shuttle had ever been launched in temperatures below 51 degrees F. before the last flight of Challenger. NASA and the boosters' manufacturer, Morton Thiokol, had conferred the night before the flight to consider the question of the cold, which had been in the 20s for more than a day.

Rogers, secretary of state during the first Nixon administration, told reporters at a late afternoon news conference that the investigation is centering on three areas: the right booster, the external tank holding liquid oxygen and liquid hydrogen, and the weather conditions at the time of the launch. He declined to comment on the Aviation Week report.

"With the exception of the orbiter itself," Rogers said, "no possibility has been exonerated."

Rogers said the commission had "frank" and lengthy discussions with NASA officials and engineers and that they had examined many photographs of the launch.