Data obtained from a salvaged Columbia data recorder shows trouble started more than a minute earlier than previously thought.
By Dr David Whitehouse
BBC News Online science editor
Hot gas had entered the leading edge of the left wing within 16 seconds of the time that Columbia experienced maximum aerodynamic heating.
The investigation has focused on the left wing
Looking at the new information, one accident investigator said that it indicated Columbia began its descent with the mechanism of failure already in place.
The analysis is of data from the OEX recorder, which was found in a Texan field on 19 March.
The OEX is situated below the crew compartments' middeck floor. It records the final 45 minutes of flight, logging data from 721 sensors situated all over the shuttle.
Although damaged, engineers have been able to recover invaluable data from its 9,400 feet of magnetic tape.
Plume of superheated air
Until the OEX readout, investigating engineers had only real-time telemetry data to help them determine the cause of Columbia's break-up on 1 February.
Columbia entered the region of maximum aerodynamic pressure at 8:50:53 EDT that fateful Saturday morning.
Using OEX information, investigators say the first sign of trouble occurred when a brake line in the left wing's main landing gear displayed an unusual rise.
The salvaged OEX data recorder
Ten minutes later Columbia broke up.
Following the first signs of trouble, a string of temperature sensors failed, probably as the result of a plume of superheated air forcing its way into the left wing.
A temperature sensor behind the left wing's leading edge - just behind the point where it is believed the problem started - shows a rapid increase before it failed.
From all the available evidence, accident investigators believe that the initial breach occurred on the underside of the leading edge of panel 6 on the left wing.
The analysis of the readings from 721 sensors recorded by the OEX has just begun and engineers are meeting on Monday to determine how to proceed.
In another move, the US National Imagery and Mapping Agency, the organisation that operates the US's spy satellites, has said it will routinely image space shuttles in orbit from now on.
Following the loss of Columbia, there was much debate because of the suggestion of possible damage to the left wing as a result of a piece of debris falling off the shuttle's external fuel tank and striking the left wing.
Nasa decided not to request any in-orbit imaging of Columbia by space or ground-based telescopes because it believed the space shuttle was in no danger and that in any case, the images would not be good enough to determine any damage.
Also, a safety panel that regularly reports on the safety of the space shuttle has said that Nasa should think about a crew escape system.